adj. Physically or chemically unaffected by high temperatures
the GNU version of the Collaborative International Dictionary of English
adj. Capable of being heated to or somewhat above 55° C. without loss of special properties; -- said of proteins, immune substances, etc.
The Century Dictionary and Cyclopedia
Unaltered in composition or activity by a moderate degree of heat: noting especially certain toxins and ferments. Opposed to thermolabilc.
Word Usage
"Even if new productions give vaccines more stable to heat (called "thermostable"), they still have to be stocked in the refrigerator between 0 and 8°C, and cold chain strictly have to be respected during transport."