gratuity vs offertory

gratuity

noun
  • An additional payment given freely as thanks for service. 

offertory

noun
  • The part of the Eucharist service when offerings of bread and wine are placed on the altar and when any collection is taken; also, the money or other things collected. 

  • A linen or silken cloth anciently used in various ceremonies connected with the administration of the Eucharist. 

  • A prayer said or sung as an anthem while offerings of bread and wine are placed on the altar during the Roman Catholic Mass or the Anglican Communion service. 

How often have the words gratuity and offertory occurred in a corpus of books? (source: Google Ngram Viewer )