The Greek word for eternal can mean "lasting forever" or "lasting for an age." It refers to "the age to come" (Matthew 13:39–40, 49; 24:3; John 17:3; 2 Thessalonians 1:9; Philemon 1:15; 1 John 5:11, 13).
Some hope that the punishment of the damned will end after "an age." God will either destroy or forgive them. Yet, the comparison between eternal life and eternal punishment strongly suggests it means "lasting forever." This is because eternal life does not end (see also Luke 16:19–31; Hebrews 6:2; Revelation 20:10).