FROM: Merriam-Webster's Online Dictionary:
* chris·mon
Pronunciation: 'kriz-"män
Function: noun
Inflected Form(s): plural chris·ma /-m&/; or chrismons
Etymology: Medieval Latin, from Latin Christus Christ + Late Latin monogramma monogram
Date: 1872
see: CHI-RHO
* Chi-RhoPronunciation: 'kI-'rO, 'kE-
Function: noun
Inflected Form(s): plural Chi-Rhos
Etymology: chi + rho
Date: 1868
: a Christian monogram and symbol formed from the first two letters X and P of the Greek word for Christ -- called also Christogram.