A choice between two alternatives; having to choose one of two options (a four-character idiom).
経営陣は二者択一を迫られ、苦しい決断を下した。
The management was forced to choose between two options and made a difficult decision.
二者択一ではなく、第三の解決策を探した方がいい。
Rather than choosing between two options, it's better to look for a third solution.
試験では二者択一の問題が多く出た。
The exam had many two-choice (binary) questions.