I don't know if there's more special word for it in general, but in the US it's known as the Connecticut Compromise
Connecticut Compromise (also known as the Great Compromise of 1787 or Sherman Compromise) was an agreement that large and small states reached during the Constitutional Convention of 1787 that in part defined the legislative structure and representation that each state would have under the United States Constitution. It retained the bicameral legislature as proposed by Roger Sherman, along with proportional representation of the states in the lower house, but required the upper house to be weighted equally among the states. Each state would have two representatives in the upper house.
Also, this form of compromise is rather common in other federations with bicameral legislature:
A federal upper house may be based on a special scheme of apportionment, as is the case in the senates of the United States and Australia, where each state is represented by an equal number of senators irrespective of the size of its population.
Alternatively, or in addition to this practice, the members of an upper house may be indirectly elected by the government or legislature of the component states, as occurred in the United States prior to 1913, or be actual members or delegates of the state governments, as, for example, is the case in the German Bundesrat and in the Council of the European Union.
In Canadian terminology, the concept is known as "intrastate federalism"
Provision for the designated representation of distinct regional views within the federal policy-making institution, usually provided by the particular form of the federal second chamber.
In that context, it is contrasted with interstate federalism, which refers to the relationship between the federal government and those of the states.
• Intrastate Federalism: The representation of the units of the federation within the national government (e.g. US Senate – 2 Senators from each State) – Constitution ensures Provincial Representation in Senate and House of Commons – Convention of regional balance in Cabinet [...]
• Interstate Federalism: Interactions between Federal and Provincial gov’ts – Joint programs – Bureaucratic linkages and consultations – Integration of Tax system – First Ministers’ Conferences
• Interstate federalism functions more smoothly, or there is simply less need for it, if intrastate mechanisms are legitimate and effective.
And if you are curious about equal vs non-equal representation of states in the upper house, again from a Canadian source, but which does pan out to a worldwide survey....
While equality of provincial representation in the Senate has not been suggested in every past constitutional proposal, it has certainly been the option of choice for latter-day reformers. (See Appendix 14, Senate Composition Schemes). Opinion-leaders like the Canada West Foundation (1981), the Alberta Legislature (1985) and Premier Clyde Wells (1989) all proposed equal provincial representation in a reformed Senate of Canada. The fact that the most consensus-oriented constitutional agreement of the last century, the Charlottetown Accord, featured equality of provincial representation in the reformed Senate as a basic principle, was telling.
The reasons for this are not hard to imagine. Populist reformers in Canada were heavily influenced by American representation theory. The founding of the American federation was greatly aided by the “Connecticut Compromise,” which joined representation by population (in the House) to equality of regional representation (in the Senate). This institutionalized system of countervailing power has seemed fair to Canadian reformers, especially those who reside in less populated provinces. Another reason is that the imbalance of (population-based) representation in the Commons is so glaring that there is little apparent incentive for governing parties based largely on representatives from Ontario and Quebec to pay much attention to the needs of the western and eastern provinces. Yet another is that the idea is simplicity itself, and Federal Representation of the People and Government of Newfoundland and Labrador simple solutions are appealing in a political system that seems to thrive on preserving arcane and complicated political forms.
As for other countries, there are differences of opinion. Ron Watts noted that equality of representation of the component units is the exception rather than the rule in federations, at least in the 10 federations he studied (See Appendix 15). However, our review shows that, if more recent data are considered and a larger number of federations (21) is considered, equality of regional representation is more the norm than is inequality (See Appendix 16). This is in spite of the fact that there are significant variations in the populations of most of the states or provinces in countries with federal systems. In Argentina, Brazil, Malaysia, Mexico, Nigeria, Russia, South Africa, USA, and Yugoslavia, for example, there is equality in the representation for the elected part of the Upper House but there are vast inequalities in state populations.
So if we were to extract some generic terminology from this, it could be "equality of regional representation", although I guess this phrase could also mean other things in other contexts.
Or more generally (i.e. not necessarily equal representation) "(second) chambers based on territorial representation". This concept can in turn be further subdivided in regional representation or direct government representation [of the regional governments], as well as mixed systems of these two flavors.
For the regional principle to prevail, seats do not have to be allocated equally across regions. What matters is the principle that is articulated in the constitution and the extent of disproportionality between seats and population. Where the constitutional principle is explicitly territorial this meets the criterion even if regions happen to be represented in rough proportion to their population. A rule of thumb for territorial representation is where the disproportion of seats per voter exceeds 5.0 between the most and least represented regions.
The German Bundesrat establishes regions as the unit of representation even though the number of seats per Land ranges from three to six. Each Land has at least three votes, and most have more in line with a constitutionally mandated population rule that gives four seats to Länder with more than two million inhabitants, five seats to Länder with more than six million, and six seats to Länder with more than seven million. The disproportion of seats to population across Länder reaches a whopping 1:13. This compares with less than 1:3 for the Italian senate. Between 1997 and 2006 each Thai changwat received between one and four seats in the senate which yields a disproportion of 1:3.5 between the most and least represented region. This is a gray case, [...] given that the Thai constitution does not articulate the territorial principle [...]
[As for a mixed example:] Since 1995, the Belgian senate comprises three kinds of community representatives: forty directly elected senators, twenty-one indirectly elected community senators, plus ten senators selected by these groups. The community senators are selected on the principle of regional representation (the Flemish and Francophone communities each have ten seats with one seat for the tiny German-speaking community) and they serve as delegates of the communities.
Also, this source notes a trend:
Most upper chambers came to serve as bulwarks against the principle of one citizen, one vote. They were conservative, sometimes reactionary, bodies representing the aristocracy, the church, corporatist groups, or territorial communities with premodern roots. Upper houses are in decline. Thirty-six of the eighty-one countries we observe had a bicameral parliament in 2010, whereas forty-three countries had one at the time they enter the dataset. Nineteen of these upper chambers represent territorial communities in 2010. [...]
