Refers to a temperature that is maintained at a fixed level, often used in scientific, industrial, or cooking contexts.
This room is kept at a constant temperature.
Please store at a constant temperature.
恒温 also means 'constant temperature' but is more technical and often used in compound terms like 恒温動物 (warm-blooded animal). 一定温度 is a more general and descriptive term.
Compound of 一定 (fixed, constant) and 温度 (temperature).