Unavoidable, a.
1. Not avoidable; incapable of being shunned or prevented; inevitable; necessary; as, unavoidable troubles.
2. (Law)
Not voidable; incapable of being made null or void. Blackstone. Unavoidable hemorrhage (Med.), hemorrhage produced by the afterbirth, or placenta, being situated over the mouth of the womb so as to require detachment before the child can be born. -- Un`a*void"a*ble*ness, n. -- Un`a*void"a*bly, adv.
Shades of meaning
Fernald has no entry of its own for unavoidable; he lists it under necessary.
Where he writes about it:
Inevitable (L. in, not, and evito, shun) is primarily the exact equivalent of the Saxon unavoidable; both words are applied to things which some at least would escape or prevent, while that which is necessary may meet with no objection; food is necessary, death is inevitable; a necessary conclusion satisfies a thinker; an inevitable conclusion silences opposition. — from necessary