• denoting the next after W in a set of items, categories, etc.
• denoting an unknown or unspecified person or thing

  "there is nothing in the data to tell us whether X causes Y or Y causes X"

• (in describing play in bridge) denoting an unspecified card other than an honour

  "East knows that declarer cannot hold a doubleton Qx"

• the first unknown quantity in an algebraic expression, usually the independent variable
• denoting the principal or horizontal axis in a system of coordinates

  "the x-axis"

"two wires in the form of an X"

• used to indicate a position on a map or diagram

  "put an X on the spot where you think its eye should be"

• used to indicate a mistake or incorrect answer
• used in a letter or message to symbolize a kiss
• used to indicate one's vote on a ballot paper

  "with trembling hand you inscribe your X on the slip of paper"

• used in place of the signature of a person who cannot write

  "nearly all those engaged in the fur trade signed with an X"