法相宗所立五性之一。聲聞，獨覺，菩薩三乘之種子皆具，或可為阿羅漢，或可為辟支佛，或可為佛，無有一定之向，故名不定性。此有四類：一具菩薩聲聞之二性，而開果不定也。二具菩薩獨覺之二性，而開果不定也。三具聲聞獨覺之二性，而開果不定也。四三乘皆具而開果不定也。此中第三者永無成佛之期，餘者時至而成佛者也。菩提心論曰：「若不定性者，無論劫限，遇緣便迴心向大，從化城起以為超三界，謂宿信佛故乃蒙諸佛菩薩加持力，而以方便遂發大心。」
(不定種性) Of indeterminate nature. The 法相宗 Dharmalakṣana school divides all beings into five classes according to their potentialities. This is one of the divisions and contains four combinations: (1) Bodhisattva-cum-śrāvaka, with uncertain result depending on the more dominant of the two; (2) bodhisattva-cum-pratyekabuddha; (3)śrāvaka-cum-pratyekabuddha; (4) the characteristcs of all three vehicles intermingled with uncertain results; the third cannot attain Buddhahood, the rest may.