What is the technical term for a "government by technology" called? By "by technology" I mean that, for example, a computer program's decisions form the basis of governance. Is this "technocracy"?
The word cybernetics comes from Greek κυβερνητική (kybernētikḗ), meaning "governance", i.e., all that are pertinent to κυβερνάω (kybernáō), the latter meaning "to steer, navigate or govern", hence κυβέρνησις (kybérnēsis), meaning "government", is the government while κυβερνήτης (kybernḗtēs) is the governor or "helmperson" of the "ship".
Cybernetics is quite a broad term, and covers a wide range of situations, but is generally whenever there is some automatic mechanism with an information based feedback loop. Scaling up to the level of socioeconomic governance would not invalidate the term, and there were some efforts along those lines as noted below.
Cybernetics and economic systems
The design of self-regulating control systems for a real-time planned economy was explored by Viktor Glushkov in the former Soviet Union during the 1960s. By the time information technology was developed enough to enable feasible economic planning based on computers, the Soviet Union and eastern bloc countries began moving away from planning and eventually collapsed.
More recent proposals for socialism involve "New Socialism", outlined by the computer scientists Paul Cockshott and Allin Cottrell, where computers determine and manage the flows and allocation of resources among socially-owned enterprises.
as well as here we have
Proposals for utilizing computer-based coordination and information technology for the coordination and optimization of resource allocation (also known as cybernetics) within an economy have been outlined by various socialists, economists and computer scientists, including Oskar Lange, the Soviet engineer Viktor Glushkov, and more recently Paul Cockshott and Allin Cottrell.
Also worth mentioning is Chile's Project Cybersyn, named from "Cybernetic Synergy", which was an attempt to increase government efficiency with a computerized decision support system, although Cybersyn would still have humans to make an ultimate decision, it is technically a computer program whose decisions help to inform governance.
There is likely no extant name for this thing
Claiming something doesn't exist is hard. I can't help much with fictional examples of this kind of government, but in studying political philosophy and political science I have never seen a non-fictional description of a government where authority is given to a computer.
To help detect examples of this idea I consulted the Stanford Encyclopedia of Philosophy and Internet Encyclopedia of Philosophy (both run by philosophy faculty and generally high-quality), but did not find anything resembling this. I also tried Google Scholar, but did not find any examples outside of fiction.
But it's not "technocracy"
In the world of non-fiction I've seen technocracy used in three cases. However, none of them involve computers or AI in any explicit way.
In general organizational theory technocracy means that an organization is governed by technical experts (Technocracy at Work, a sociology book, has a great definition section which discusses this idea). The idea is that their technical expertise is what qualifies them to govern.
There is a rare usage of the term in political science which is somewhat similar. Governments where high-ranking administrators are experienced members of their department with significant technical experience can be called "technocracies". This is sometimes used to distinguish the United States (where cabinet level officials are not technical experts) from places like France (Ridley, 1966).
Finally, there is an extant technocracy movement, at least in the United States. Following from the same premise as the above examples, their major issue is a rejection of price-based economics, which they believe to be an example of inefficiency. Instead, technical experts should be capable of directing economic and social programs to maximize efficiency.
Computer programs are tools, they don't decide things outside of how they were programmed (or how they were programmed to learn and then taught, for some kinds of machine-learning - still highly guided by the programmer though). There's no technology that is independent of a designer, which means that the "technology making the decision" is really "a person making the decision via technology". It's a level of indirection, but it's still controlled.
Kind of like when airlines have said that spikes in ticket prices due to emergencies aren't their fault because it's just an algorithm ignoring that they control the algorithm.
While that could change in the future, as it is there is no real way for this to be a possibility.
However, science fiction has been exploring this topic for decades - one of the earliest examples is from Asimov, in a short story called Evidence in 1946, and its follow-up The Inevitable Conflict in 1950.
It seems like this is just a specialized form of autocracy. Perhaps you could call it an "AI Autocracy." I'm assuming, of course, that the government enforces the decisions of the computer without question, but you can take pretty much any form of government where a central power has the power to both decide on and execute laws and insert your AI, and the type of government is still the same to the outside observer.