Wiktionary
dominus vobiscum
interj. (context Roman Catholicism English) A devout salutation often used as a prelude to certain set prayers; the traditional response is ''Et cum spiritu tuo''.
Wikipedia
Dominus vobiscum
Dominus vobiscum ( Latin: "The Lord be with you") is an ancient salutation and blessing traditionally used by the clergy in the Roman Catholic Mass and other liturgies, as well as liturgies of other Western Christian denominations.