As nouns the difference between cos and tan

is that cos is a variety of lettuce with long, crisp leaves while tan is a yellowish-brown colour.

As proper nouns the difference between cos and tan

is that cos is an island of the Dodecanese, Greece while Tan is an English surname derived from the transliteration of various Asian surnames, particularly originating from China and Vietnam.

As a conjunction cos

is because.

As an adjective tan is

of a yellowish-brown.

As a verb tan is

to change to a tan colour due to exposure to the sun.

As a numeral tan is

the second cardinal number two, formerly used in Celtic areas, especially Cumbria and parts of Yorkshire, for counting sheep, and stitches in knitting.See Wikipedia article on Yan Tan Tethera

As an initialism TAN is

Total Acid Number, the measure of a lubricant's or crude oil's acidity.