The Phoenician letter name ṭēth means "wheel", but the letter possibly (according to Brian Colless) continues a Middle Bronze Age glyph named ṭab "good", Tav in Aramaic and Tov טוב in Hebrew, ṭayyib طَيّب in modern Arabic, based on the nfr "good" hieroglyph:
Jewish scripture books about the "holy letters" from the 10th century and on discuss the connection or origin of the letter Teth with the word Tov, and the Bible uses the word 'Tov' in alphabetic chapters to depict the letter.
In gematria, Tet represents the number nine. When followed by an apostrophe, it means 9,000. The most common example of this usage is in the numbers of the Hebrew years (e.g., ט'תשנד in numbers would be the date 9754).
As well, in gematria, the number 15 is written with Tet and Vav, (9+6) to avoid the normal construction Yud and Hei (10+5) which spells a name of God. Similarly, 16 is written with Tet and Zayin (9+7) instead of Yud and Vav (10+6) to avoid spelling part of the Tetragrammaton.
A symbol similar to the Phoenician teth is used for the tensor product, as , but this is presumably an independent development, by modification of the multiplication sign ×. The Hebrew ט is also visually similar to the letter Ʋ.
^This was especially emphasized ever since the late 1600s after the Baal Shem Tov became influential, since the letter Teth was in his Acronym standing for Tov, and goodness was part of his philosophy.