The Greek adjective aphthartos means imperishable, incorruptible, or not subject to decay. It derives from a- (negative) + phtheirō (to corrupt, destroy). It describes that which is exempt from the decay and corruption that marks the fallen creation.
Aphthartos is a key eschatological word in Paul's resurrection theology. The mortal must 'put on immortality' (aphtharsia) — the resurrection body is raised aphthartos (imperishable: 1 Corinthians 15:42, 52-54). This imperishability is not a quality natural to humans but received by grace through resurrection. The 'inheritance that can never perish' (aphthartos) of 1 Peter 1:4 is kept in heaven for believers. God Himself is aphthartos (Romans 1:23; 1 Timothy 1:17) — the living God who cannot decay stands in infinite contrast to idols that perish. The entire Christian hope is anchored in this indestructible quality of God and His promises.