ΙΧΘΥΣ (ichthus) is the Greek word for fish used in passages such as (Matt. 7:10; 14:17; Mark 6:38, 41; Luke 5:6; 11:11; John 21:6, 8, 11).  Today it is best known as the "Christian fish" symbol popularized on the backs of cars and on some Christian business logos.  It has also been used as an acronym for truths about Jesus Christ.

Ι - for Ἰησοῦς (Iesous) or Jesus.  The name given to Mary's baby boy (Luke 1:31)

Χ - for Χριστός (Xristos) or Christ. The angel's announced to the shepherds, "He is Christ the Lord" (Luke 2:11).

Θ - for θεός (theos) or God. 
Υ - for υἱός (huios) or Son. These two letters together represent the title "Son of God" (Luke 1:25).

Σ - for σωτήρ (soter) or Savior.  Another title given to Jesus at his birth (Luke 2:11).

Taken together, He is Jesus Christ, the Son of God, the Savior of the world.

The lowercase, fully accented Greek lettering is ἰχθύς , but the word is usually found in all uppercase without accents (ΙΧΘΥΣ) in order to emphasize its use as an acronym.