Opposition, Opposite
noun, な adjective, の adjective, verbal noun
The anti versus... You'd think this would be a double negative, but it's like a lot of other words where you have two similar words combined... it just means the same thing as those two words. In this case, the anti-versus of something would be the opposite. Alternatively, it can be the opposition. You choose.
はんたい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
その考えには反対です。
I’m opposed to the idea.
あの、そのサンダル、右と左が反対ですよ。
Um, your right and left sandals are on opposite feet.
父は、私がウォルマートでプロポーズすることを反対した。
My father was opposed to me proposing in a Walmart.