fixed income
Refers to a regular, stable income that does not fluctuate, such as a salary or pension.
定収入があると生活が安定する。
Having a fixed income stabilizes your life.
収入 is general 'income'; 定収入 specifically means a fixed, regular income.
固定収入 is a synonym for 'fixed income', but 定収入 is slightly more formal and less common.
Compound of 定 (fixed) and 収入 (income).