|Which is correct?
Although ISPs promote DNS redirection as a service to customers, customers typically think the ISPs have _______ motives.
Explanation: Ulterior, in this context, means hidden. Alterior, on the other hand, is not a word. If it were, however, it probably would mean something like alternate. Which is probably where the confusion comes from — people thinking that saying someone has “an alterior motive” means they have a different motive from the one they claim.
Alterior is featured in the Eggcorn Database. (An eggcorn is a word used mistakenly because it sounds like the correct word.)
Grammar Girl differentiates between Spoonerisms, Mondegreens, Eggcorns, and Malapropisms.