Pon farr

Pon farr /ˌpɒn ˈfɑːr/ is a phenomenon in the fictional Star Trek universe. A part of the reproductive cycle of Vulcans, pon farr features in the canonical TV series as well as in various spin-offs and fan fiction.

Every seven years, Vulcan males and females become aroused. They undergo a blood fever, become violent, and finally die unless they mate with someone with whom they are empathically bonded or engage in a ritual battle known as kal-if-fee.

A common misconception associated with the series (and Spock in particular) is that Vulcans only have sex once every seven years. However, pon farr is not coincident with the sex lives of Vulcans, and they are able to have intercourse without the affliction, and thus more than once every seven years. Star Trek: The Original Series writer and continuity story editor D. C. Fontana explains that pon farr is not the only time that Vulcans feel sexual desire or engage in sexual activity:

Vulcans mate normally any time they want to. However, every seven years you do the ritual, the ceremony, the whole thing. The biological urge. You must, but any other time is any other emotion—humanoid emotion—when you're in love. When you want to, you know when the urge is there, you do it. This every-seven-years business was taken too literally by too many people who don't stop and understand. We didn't mean it only every seven years. I mean, every seven years would be a little bad, and it would not explain the Vulcans of many different ages which are not seven years apart.

Vulcans not only are able to mate outside of pon farr, they are also able to mate with species other than Vulcan e.g., in Star Trek: The Original Series, Spock's parents are human/vulcan couple; In Star Trek: Enterprise, T'pol, a vulcan, has a romantic and sexual relationship with Trip, a human; in the Star Trek reboot, Spock is in a relationship with Lt. Uhura.

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