At least 400 EVs and at least an 9-point or 10-point margin between the two candidates, such as Bush vs Dukakis in '88 or Reagan vs Carter in '80. ('84 was a given).
1988 a landslide ? LOL
Well given my standards of at least 400 electoral votes then yes it was a landslide.
Many of the thresholds proposed here seem to make an awful lot of landslide elections. Maybe posters think that either an election is close or a landslide with very little room in between. I think that both very close elections and landslides are at the edges of the spectrum and that comfortably large wins should be the norm when elections are graded.
I think the best idea is to determine what elections are landslides, then set a threshold so that only those landslide elections pass the cut. For instance, I wouldn't call the 1988 election a landslide nor would most observers who watched it. It was a solid win, but that doesn't make it a landslide.