Generally, Christen and Baptise mean exactly the same thing. "Christen" actually means "to make Christian" or "to bring to Christ" but most dictionaries define "Christen" and "Baptise" as exactly the same, and this is the common understanding.
When we had our first born 'christened' the vicar (CofE, not Catholic) scalded us for using the term 'christening' as it's a non-religious saying, apparently. Whether CofE or Catholic, the Church refers to it as baptism.