A β-hydrogen, or beta hydrogen, is a hydrogen atom on the carbon directly adjacent to the carbon that bears the leaving group. It matters because its removal, together with departure of the leaving group, is what creates the new carbon–carbon double bond (C=C) in an elimination. A substrate must possess at least one β-hydrogen in order to undergo elimination; the carbon bearing the leaving group is the α-carbon, and the neighbouring carbon supplying the hydrogen is the β-carbon.

β-氢(贝塔氢)是位于直接连有离去基团的碳的相邻碳上的氢原子。它之所以重要,是因为脱去它并伴随离去基团的离去,正是在消除反应中生成新碳碳双键(C=C)的原因。底物必须至少含有一个 β-氢才能发生消除;连有离去基团的碳为 α 碳,提供该氢的相邻碳为 β 碳。