Real Dictionary

X
When writing down the size of something, you can use x in between the measurements to mean 'by'.
variable noun: X is the twenty-fourth letter of the English alphabet.

X in American English
noun: the 24th letter of the English alphabet, a consonant

X in American English 1
a mark shaped like an X used:
the twenty-fourth letter of the English alphabet: from a western form of the Greek alphabet
abbreviation: experimental
transitive verb: to cross out or mark with or as if with an x

X in American English 2
Christ
to indicate (one's choice or answer) by or as by marking with an X
abbreviation
symbol: the 24th in order or in a series, or, when I is omitted, the 23rd

X in American English 3
reactance
the first of a set of unknown quantities, y usually being second
symbol: an unknown quantity or a variable

x in American English 4
extra

x in American English 5
by (in indicating dimensions)

x in British English
noun: the 24th letter and 19th consonant of the modern English alphabet
symbol for: ex
symbol for: indicating a film that may not be publicly shown to anyone under 18. Since 1982 replaced by symbol 18