ALTERIOR is NOT a Word. You must be thinking of ULTERIOR...

A simple mistake to make because there is little difference between the pronunciation of alterior and ulterior, many English learners and native speakers assume that alterior is actually a word meaning alternate or alternative when in fact, it is not.

Often we hear phrases such as:

WRONG:

"Be careful with that client because he has alterior motives."
(The man has different or alternative motives.)

RIGHT:

"In marrying him, she had ulterior motives other than true love."
(The woman has secret or hidden motives, such as money or status.)

As you can see in these examples, the correct word is ulterior with a 'u', which means secret or hidden motives or intentions.  It does not mean other or alternative motives.  So the next time you're writing, don't forget to use ULTERIOR rather than ALTERIOR!