The mex of a subset of a well-ordered set is the smallest value from the whole set that does not belong to the subset.
"In her lecture on ordinal arithmetic, the professor explained how to calculate the mex by identifying the least natural number absent from the given sequence."
Abbreviation of México. (a state of Mexico)
"In casual texting, he signed his name as mex instead of the more common Mex."
Alternative form of Mex.