A choice between two options; a binary choice (either-or).
問題は二択だから、答えはどちらか一つだ。
It's a two-choice question, so you must pick one of the answers.
行くか行かないかの二択を迫られた。
I was forced to choose between going or not going.
アンケートは二択式で『はい／いいえ』だけだった。
The survey was binary: only 'yes/no'.