What is the X in X-mas?
Roman letter, from Greek lambda, from Semitic lamed. The shape of the Roman letter is an early one in Greek, adopted before it was superseded in Greek by the inverted form which became the Greek lambda.
limes (used with a lower case letter or a plus sign, or used with a subscript letter or plus sign as a symbol for various doses of toxin)
or l liter
Abbreviation of length, liter