What is the X in X-mas?
early 15c., "one and indivisible" (with reference to the Trinity), from Medieval Latin individualis, from Latin individuus "indivisible," from in- "not, opposite of" (see in- (1)) + dividuus "divisible," from dividere "divide" (see divide). Not common before c.1600 and the 15c. usage might be isolated. Sense of "single, separate" is 1610s; meaning "intended for one person" is from 1889.
"single object or thing," c.1600, from individual (adj.). Colloquial sense of "person" is attested from 1742. Latin individuum meant "an atom, indivisible particle;" in Middle English individuum was used in sense of "individual member of a species" from early 15c.