UNIQUE WORDS game to increase our vocabulary - alphabetical order

Retronym - a new term created from an existing word in order to distinguish it from the meaning that has emerged through progress or technological development (e.g., cloth diaper is a retronym necessitated by the fact that diaper now more commonly refers to a disposable diaper).

Tontine - 

an annuity shared by subscribers to a loan or common fund, the shares increasing as subscribers die until the last survivor enjoys the whole income

• a scheme for life insurance in which the beneficiaries are those who survive and maintain a policy to the end of a given period

Vehemence - the display of strong feeling; passion.

XO - Executive Officer (military), second in command

Zoomer - is a nickname referring to members of Generation Z, those born in the late 90s and early 2000s

behoove
be appropriate or necessary

dearth
an insufficient quantity or number

Fenestron - is a protected tail rotor of a helicopter operating like a ducted fan