According to Dictionary.com:
Adjective-
1)Unable to be avoided, evaded, or escaped; certain; necessary: an inevitable conclusion.
2)Sure to occur, happen, or come; unalterable: The inevitable end of human life is death.
Noun-
1)That which is unavoidable.
Accept that some things are inevitable. This is not to say one should throw up their hands and write off everything as inevitable, but rather realize that some things are easy to change, others are possible, some are difficult, and some are inevitable, at least temporarily. Recognize what cannot be changed and try not to waste time or energy worrying over such things. In regards to people and acceptance, realize that some aspects of people are inevitable, and in order to accept them, these unmovable aspects must also be accepted.