"IHT" on a Christian cross is an abbreviation of the first three letters of the name of Jesus in Greek:
ΙΗΣΟΥΣ (Iēsous).
* Ι (Iota) - corresponds to "I"
* Η (Eta) - corresponds to "E"
* Σ (Sigma) - corresponds to "S"
Therefore, IHT is a shortened form of the sacred name Jesus in Greek. Sometimes, it might appear as IHS, especially in Latin contexts.