A device for measuring time.

24-hour clock

A method of keeping time in which the 24 hours of the day are numbered from 00:00 (or 0000) though 23:00 (or 2300), where 00:00 refers to midnight and the last minute of the day is 23:59. In the medical professions, such notation helps prevent ambiguity in, e.g., documentation of patient care on charts.
SYN: SEE: military time

biological clock

1. An internal system in organisms that influences behavior in a rhythmic manner. Functions such as growth, feeding, secretion of hormones, the rate of drug action, the wake-sleep cycle, the menstrual cycle, and reproduction coincide with certain external events such as day and night, the tides, and the seasons. Biological clocks appear to be set by environmental conditions in some animals, but if these animals are isolated from their environment they continue to function according to the usual rhythm. A gradual change in environment does produce a gradual change in the timing of the biological clock.
SEE: circadian; SEE: maladaptation to night work; SEE: zeitgeber
2. A colloquial term for the decrease in fertility that accompanies aging, particularly as women approach the age of 35.