K (named kay /ˈkeɪ/) is the eleventh letter of the ISO basic Latin alphabet.

In English, the letter K usually represents the voiceless velar plosive; this sound is also transcribed by /k/ in the International Phonetic Alphabet and X-SAMPA.

The letter K comes from the Greek letter Κ (kappa), which was taken from the Semitic kap, the symbol for an open hand. This, in turn, was likely adapted by Semites who had lived in Egypt from the hieroglyph for "hand" representing D in the Egyptian word for hand, d-r-t. The Semites evidently assigned it the sound value /k/ instead, because their word for hand started with that sound. In modern-day English slang, the word "k" is used as a substitute for the abbreviation "O.K.", or "Okay." In International Morse code it is used to mean "over".

In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the sounds /k/ and /g/ (which were not differentiated in writing). Of these, Q was used to represent /k/ or /g/ before a rounded vowel, K before /a/, and C elsewhere. Later, the use of C (and its variant G) replaced most usages of K and Q. K survived only in a few fossilized forms such as Kalendae, "the calends".

Baby K (October 13, 1992 – April 5, 1995) was an anencephalic baby who became the center of a major U.S. court case and a debate among bioethicists.

Stephanie Keene, better known by the pseudonym Baby K, was born at Fairfax Hospital in Virginia, USA. At the time of her birth, she was missing most of her brain, including the cortex; only the brainstem had developed during pregnancy, the portion of the brain responsible for autonomic and regulatory functions, such as the control of respiration, the heartbeat and blood pressure.

The baby's mother had been notified of her condition following an ultrasound, and had been advised to terminate the pregnancy by her obstetrician and neonatologist but chose to carry the child to term because of "a firm Christian faith that all life should be protected". The hospital's viewpoint was that care provided to the baby would be futile. The mother's viewpoint was that mechanical breathing support must be provided during the baby's periodic respiratory crises. Fairfax Hospital doctors strongly advised a do not resuscitate order for the child, which the mother refused. Stephanie remained on ventilator support for 6 weeks while Fairfax searched for another hospital to which to transfer, but no other hospital was willing to accept her. After the baby was weaned off constant ventilator support, the mother agreed to move the child to a nursing facility, but the baby returned to the hospital many times for respiratory problems.