Definitions:

1. Denoting an unknown or unspecified person or thing. NOUN
   • There is nothing in the data to tell us whether 'X' causes Y or Y causes 'X'.
2. Denoting the principal or horizontal axis in a system of coordinates. NOUN
   • The X-axis represents the probability threshold.
3. In mathematics an 'X' represents multiplication. NOUN
   • 2x5=10
4. the 24th letter of the Roman alphabet noun
   • X comes before Y in the alphabet.
5. The first unknown quantity in an algebraic expression, usually the independent variable. NOUN
6. The Roman numeral for 10. NOUN
   • XX=20
7. Used in a letter or message to symbolize a kiss. NOUN
8. Used in place of a signature of a person who cannot write. NOUN
   • Nearly all those engaged in the fur trade signed with an 'X'.
9. Used to indicate a mistake or incorrect answer. NOUN
   • The professor took his pen and wrote a large 'X' upon the sheet of paper.
10. Used to indicate a position on a map or diagram. NOUN
    • Put an 'X' on the spot where you think its eye should be.

Synonyms:

• ex
• ten