There's a sense missing from the
Webster 1913 list below, and it's a very common sense in the USA though not in the rest of the English-speaking world.
The word alternate whether adjective or verb basically means switching from one to the other and back again. (verb) A zebra's stripes alternate between black and white. (adjective) Alternate stripes are black and white.
Webster didn't mention the new sense so it must have developed in America since 1913. In American English, the adjective alternate is used to mean alternative, that is a choice. Here is one route to Boston, and here is an alternative route -- or in America, an alternate route.
This change of meaning is a bit surprising because it compresses two words into one. One of the meanings has been lost. How do you say that a zebra's stripes are alternately black and white?
The pronunciation is another minefield. My dictionaries give a choice including AL- as in Al Capone. But I've never heard this. The alternatives are short ol- as in holly and long awl- as in awl and all. Of course in America those are both the same "ahl" sound.
Then the stress. The verb is OL-ter-nayt, with initial stress and a full "nayt" vowel at the end. The adjective however is either OL-t'-n't to match that, or ol-TER-n't to match the adjective ol-TER-n't-iv.