Betazoid phase
The Betazoid phase or more commonly known as "the Phase" was a physiological phase that all Betazoid and half-Betazoid females entered during mid-life, similar to Human menopause. It was only during "the Phase" that a Betazoid female became fully sexual as the sex drive of the female in question quadrupled or more. Some females engaged in sexual relationships with any male available, while others focused their attention on a single male who would later become her husband. In the event that a male spurned the female's advances, the female would be deeply offended and take the rejection personally. In some cases, the female's telepathic abilities were impeded by "the Phase".

At the beginning of her relationship with William T. Riker, Deanna Troi "warned" Will of "the Phase", although she did not tell him that her sex drive could increase by more than four times, not wanting to frighten him.

In 2365, Lwaxana Troi entered "the Phase", making advances on William Riker, Jean-Luc Picard as well as a hologram, Rex. It apparently had multiple stages, as Deanna twice asked Lwaxana what stage it was in, only to not receive a straight answer. ()

Deanna Troi experienced symptoms that could be attributed to the Phase in 2369 after a telepathic link to Ambassador Ves Alkar caused her to artificially age. Troi displayed an overt sexual aggressiveness and even attempted to seduce several members of the Enterprise crew. ()

See also 
Pon farr, a similar physiological phase for Vulcans

de:Phase der Umwandlung
Medical conditions