X

X is the 24th letter of the English language

 

X is the 3rd least popular letter by usage, and it has the lowest number of words starting with it

 

X marks the spot of the treasure

 

X represents the signature of one who cannot write

 

x is the multiplication sign

 

X is a kiss

 

X mark indicates negation (no, this is wrong) or affirmation (yes,vote for this)

 

X was the name of the cowboy on Sesame Street who used to bother the nice people of Sniddler's Gulch www.youtube.com/watch?v=ChahP31qh9k

Done