Literally, any hidden estrus. A property of some animals, and also humans.
In typical (non-cryptic) estrus, a female has very obvious indicators that estrus is occuring, ie flushed, enlarged genitals and buttocks, vaginal moisture, and altered behaviour.
Cryptic, or hidden, estrus, has some interesting implications for social animals. Since it is impossible to determine whether or not the female is ovulating, the male cannot have any assurance about the success of his attempts at mating. Thus, in species with cryptic estrus, males tend to mate multiple times with a female, over a period of time. This also alters many courtship and socio-sexual heirarchy rituals.
Human females may also display what is known as pseudo-estrus.