"Of one mind"
Lookups for unanimous spiked on May 10th, following reports that Stephen Curry, point guard for the Golden State Warriors, had become the first player in NBA history to be unanimously chosen as that league’s most valuable player.
Stephen Curry Is M.V.P., and This Time It’s Unanimous
—New York Times, 10 May 2016
Golden State Warriors guard Stephen Curry became the first unanimous NBA Most Valuable Player on Tuesday, winning the award for a second straight season.
—ESPN.com, 10 May 2016
Although most of the instances in which one encounters an English word beginning with un- carry the meaning of negation (unlikely, unlikeable, etc.), this is not the case with unanimous. That is because the word comes from the Latin unanimus, which itself is formed by combining unus ("one") and animus ("mind"); hence the word has taken in English the meaning "being of one mind." We see evidence of this Latin word unus elsewhere in English with words such as union (which comes from the Latin unio, meaning "oneness," and which is also from unus) and in phrases borrowed directly from Latin, such as the motto of the United States, e pluribus unum ("out of many, one").
And in case you were wondering what article should go before the word, it is more common to use a than an ("there was a unanimous decision"); although unanimous begins with a vowel, it is a Y sound, and so is preceded by a, rather than an.