pen·te·cos·ta·ri·on \ˌ ̷ ̷ ̷ ̷ˌkȯˈsta(a)rēˌän, -käˌ-\noun (-s) Etymology: Late Greek pentēkostarion, from pentēkostē Pentecost : a liturgical book in the Eastern Church containing offices for the period from Easter Sunday to the first Sunday after Pentecost