Literally 'being alongside' — the arrival and subsequent presence of a person. In secular Greek it described the official visit of a king or emperor to a city. The NT adopted it as the technical term for Christ's second coming — his royal return in glory to judge and to save.
The parousia is the great hope of the early church. Paul's earliest letters (1-2 Thessalonians) address anxious questions about what happens before and during the parousia. Matthew 24 is Jesus's extended discourse on the signs preceding his parousia. James 5:7-8 uses it for pastoral encouragement: 'Be patient, therefore, brothers, until the parousia of the Lord.' The parousia is not escapism but the ground of ethical urgency — because he is coming, live holy lives now (2 Pet 3:11-12).