biological clock noun2 •Any of several mechanisms that regulate the periodicity of biological functions, such as the circadian rhythm.Your biological clock is actually a tiny bundle of cells in your brain that responds to light signals received through your eyes. — 2005 •(figurative) The perceived time limit of human fertility, especially for women, evoking pressure to conceive children before reproductive ability declines with age.Her biological clock is ticking. formsbiological clocks
