Question: "What is the Parousia?"
Answer: The Greek word parousia (pronounced pair-oo-see-ah) is a noun that means "a coming" or "a presence." As it is used in the New Testament, it can refer to any individual's coming or presence to a specific place or to be with specific people. For example, in 1 Corinthians 16:17, Paul writes of "the coming of Stephanas and Fortunatus and Achaicus." In 2 Corinthians 7:6, Paul writes of "the coming of Titus." In Philippians 2:12, it is used to refer to Paul's presence.
Most importantly, however, if you were to do a search of the Greek New Testament, you would find that primarily this word refers to the Coming of the Lord Jesus. It can refer to either His second coming at the END of the 7-year tribulation period (Matthew 24:27, 37, 39; Revelation 19:11 ff), or to His coming to rapture His Church PRIOR to the 7-year period known as the tribulation (1 Corinthians 15:23; 1 Thessalonians 2:19; 5:23; James 5:8; 1 John 2:28).
So, the Parousia is the coming of the Lord Jesus Christ, and you have to look at the context to determine whether it refers to His appearing in the air to rapture or catch away the Church (1 Thessalonians 4:13-18), or whether it refers to His second coming to Earth to setup His millennial kingdom (Matthew 24-25; Revelation 19:11ff; 20:4-6).