From a- (without) + sthenos (strength) — lack of strength, weakness. It covers physical illness, human frailty, and spiritual limitation. Astheneia is the common NT word for sickness and the broader concept of human weakness before God.
Paul's revolutionary claim is that divine power is perfected in human astheneia: 'When I am weak, then I am strong' (2 Corinthians 12:10). The cross itself appeared as ultimate weakness but was God's power. Jesus 'was crucified in weakness, yet lives by God's power' (2 Corinthians 13:4). Isaiah 53 prophesied a servant who bore our weaknesses. Christianity alone among religions makes weakness the vehicle of divine strength.