From peri (around) and leipō (to leave). To leave around, to remain, to survive. Paul uses it of believers who are still alive at Christ's return.
Paul assures the Thessalonians that those who are still alive at Christ's coming will not precede those who have died (1 Thessalonians 4:15, 17). The word guarantees that no believer is disadvantaged by timing — whether dead or alive at Christ's return, all will share in the resurrection glory.