To avoid confusion, the simplest thing to do is use the word baptism. It is the sacrament of baptism that is being performed and thus baptism is the most accurate word to use.
Christening has become more and more popular of a word to use and most often it refers to the baptism ceremony. Traditionally, christening refers to the name that is given to the child (given a Christian name), which takes place during the baptismal rite.
However if someone was to use the word christen to refer to infant baptism he would not be incorrect since the words are essentially used interchangeably today.
