"Baptism " is the word used in the Church of England
prayer book for the ceremony whether in home or hospital of sickly infants who may die or the more usual
baptism at the font in church at the earliest public
Sunday service possible [although in earlier times mothers
were " churched " six weeks after delivery---& therefore
could have been excluded from such an earlier ceremony.]
Baptism is still the word used for the ceremony for those
of " riper years" ---adults who chose to convert or were
found to be unbaptised[often illegitimate] when seeking marriage. I wonder if the term " Christened " became popular when infants were no longer immersed but named
& a cross marked on the forehead with holy water.Churches which practice adult baptism by immersion
do not I believe ever use " Christening"