Paternoster is an English word which is a fossilisation of two good Latin words: pater (or 'father') and noster (or 'our'). These are the first two words, and therefore the name, of a prayer which Jesus taught his disciples: the Lord's Prayer. This is probably the best-known of all Christian prayers. In the days before the Reformation when all religious services were conducted in ecclesiastical Latin, the paternoster was a piece of Latin known to most people. Hence it has been adopted into English unchanged - apart from writing it as a single word. The word has been used in other ways. It sometimes indicates generally a prayer - specifically the Lord's Prayer, or more generally any prayer - or the time it would take to say the Lord's Prayer; it has been used as an address, or street name. It is even a particular item of fishing tackle. Some meanings common in the past reflect the idea of repetitiveness built into a form of prayer the recitation of which a set number of times was sometimes enjoined on penitents by priests as a penance.
In English, it is usually pronounced to rhyme with 'latter (or later)-cost-a' IPA: /ˌpæ (or eɪ)t ər ˈnɒst ər/. In classical Latin, it would have been closer to the sound of 'martyr-cost-a' IPA: /ˌpɑːt ər ˈnɒst ər/.