Samekh or Simketh is the fifteenth letter in many Semitic alphabets, including Phoenician, Hebrew, and Aramaic, representing /s/. The Arabic alphabet, however, uses a letter based on Phoenician šin to represent /s/ (see there); however, that glyph takes Samekh's place in the traditional Abjadi order of the Arabic alphabet.
The Phoenician letter gave rise to the GreekXi (Ξ, ξ).
Samekh and Mem form the abbreviation for the Angel of Death, whose name in Hebrew is Samael. It also stands for centimetre.
In some legends, samekh is said to have been a miracle of the Ten Commandments. Exodus 32:15 records that the tablets "were written on both their sides." The Jerusalem Talmud interprets this as meaning that the inscription went through the full thickness of the tablets. The stone in the center parts of the letters ayin and teth should have fallen out, as it was not connected to the rest of the tablet, but it miraculously remained in place. The Babylonian Talmud (tractate Shabbat 104a), on the other hand, attributes this instead to samekh, but samekh did not have such a hollow form in the sacred Paleo-Hebrew alphabet that would presumably have been used for the tablets. However, this would be appropriate for the Rabbis which maintained that the Torah or the Ten Commandments were given in the later Hebrew "Assyrian" script (Sanhedrin 21b-22a).