Prefix

1. Against, hostile to.

   antiabortion

2. Opposite of, reverse.

   antimatter

   anticlockwise

3. Counteracting, neutralizing.

   antibacterial

Usage notes

• Anti- should not be confused with ante-, which is a prefix of Latin origin meaning before. (However, anti- does exist as a variant spelling of ante- in some borrowed words, such as anticipate and antipasto, but this spelling cannot be used to coin English words.)
• Anti- sometimes becomes ant- before words beginning in a vowel.

  antacid