Extra and X

Extra noun - An interchangeable part or piece of equipment that is kept on hand for replacement of an original.

X and extra are semantically related. in addition topic. In some cases you can use "X" instead the word "Extra" as a noun or an adjective or an adverb, when it comes to topics like surplus, additional, in addition, exceptionally.

X noun - The cardinal number that is the sum of nine and one; the base of the decimal system.

Extra and x are semantically related. Sometimes you can replace term "Extra" with "X", this nouns are similar.