regular pay; fixed salary
A salary or wage that is fixed and does not vary with performance or hours, as opposed to commission or overtime pay.
At this company, in addition to a fixed salary, bonuses are paid based on performance.
月給 is a monthly salary, which can be fixed or variable; 固定給 specifically emphasizes the fixed nature.
Compound of 固定 (こてい, 'fixed') and 給 (きゅう, 'pay, salary').