Crasis (Greek, ‘mixture’) is the contraction of two or more adjacent vowels across a syllable or even word boundary into one syllable. This entails vowel reduction. Crasis across a word boundary is a case of synaloepha. It is the choice process to avoid a hiatus.

There are different types of crasis:

• One vowel may be dominant, the other recessive. Then:
  • The recessive vowel may be reduced to a semivowel; this is is synizesis.
  • The recessive vowel may be lost.
• The two vowels may merge into one, usually long, vowel.

Example with vowel loss: Ancient Greek τιμάω (timáō) → τιμῶ (timô) ‘I honor’.