Is there a word for majority wherein someone or something gets all the votes cast?
unanimity juːnəˈnɪmɪti/ noun agreement by all people involved; consensus.
(definition from Google)
I think that would be "unanimous".
Unanimity is where everyone involved votes for a candidate or proposal. All are of one mind.
There may be a situation where all votes cast are for a proposal, but not everyone votes. Thus there are no votes against the proposal. This is usually indicated by nem con, for nemo contra, "no-one against".