The carbenium ion stability order describes the trend in the stability, and hence ease of formation, of carbenium ions: tertiary is more stable than secondary, which is more stable than primary, which is more stable than the methyl cation. It matters because a more stable cation forms more readily, and this order therefore governs which substrates react by the SN1 (and E1) mechanism. Cations that are resonance-delocalised, such as allylic and benzylic cations, are stabilised still further.

碳正离子稳定性顺序描述碳正离子稳定性(从而生成难易)的规律:叔碳正离子比仲碳正离子稳定,仲碳正离子比伯碳正离子稳定,伯碳正离子又比甲基碳正离子稳定。它之所以重要,是因为越稳定的阳离子越容易生成,故这一顺序决定哪些底物按 SN1(及 E1)机理反应。共振离域的阳离子,如烯丙基和苄基碳正离子,则更为稳定。