X mark

Noun

 * 1) A symbol composed of two perpendicular lines (✗), especially when representing negation or lack of verification.

Usage notes
Unicode provides two X marks, one normal and other boldened: ✗ and ✘, respectively. However, alternatively, any other similar character can be used for their purpose, such as: X, x, ×, X, ✕, ☓, ✖, etc.

Antonyms

 * (✓)
 * (◯, used in Japanese)