Although it differs from nation to nation because of differences in election practices, a Landslide Victory occurs when a candidate receives a Super-majority of the votes (2/3rds or 66%-67% (rounded, though most people will say 66% is enough of a gap, despite being slightly short) or better) (source). It's important to distinguish that these victories are not pegged to popular vote as the determination comes from the actual vote that matters, so while having a super-majority in a popular vote does help, it can be negligible in systems where the candidate is determined by a narrower field, such as a Parliamentary system, in which the PM is determined by the winning party's leadership or the vote in the House for the PM or The U.S., which uses an Electoral College, generally will declare Landslides by the Electoral College vote, even when it is not a Landslide by Popular vote (for example, Bill Clinton's Presidential elections are both considered a Landslide victory despite Bill Clinton failing to secure even simple majority of the popular vote in both cases (43% in 1992 and 49.2% in 1996)).
Keep in mind, though, that some nations do not use a 2/3rds majority as the Super-majority demarcation point and will substitute lower levels, with 60% or as low as 55% (in Colorado) as the point of Super-majority (though Colorado uses it only for State Constitutional amendments by Referendum) (source).
For a rather common term, I did find the Wikipedia page for Landslide Victories to be rather lacking on the discussion of the topic, as it only lists recent examples of such victories in the UK and discusses nothing in other nations. For a better discussion of some real life Landslides and the issues that factored into the success, I highly recommend this TVTropes article. The elections here limited to a 70% of the vote threshold unless otherwise notable (which might run counter to the Clinton example, as he never got this much in his elections, though his status as not clearing 50% of popular vote might be notable to include) to better manage the size and scope of the definition. They also list "Show Elections" where a despot holds an illiberal election to demonstrate "Popular Support".