What is the X in X-mas?
mid-15c., "act or process of increasing," from Latin incrementum "growth, increase; an addition," from stem of increscere "to grow in or upon" (see increase). Meaning "amount of increase" first attested 1630s.
increment in·cre·ment (ĭn'krə-mənt, ĭng'-)
The process of increasing in number, size, quantity, or extent.
Something added or gained.
A small positive or negative change in the value of a variable.