Eternal can be defined as...Quantitative (durational)- 1) without beginning and end, that which has been and always will be 2) without beginning 3) without end, never to cease
Qualitative (quality of life) - of the age to come*
It is easy to see examples of the quantitative aspect in the many verses that call God eternal and those that refer to eternal life.
For an example of the qualitative aspect, let's look at a verse that traditionalists and conditionalists alike claim to support their view:
“These will pay the penalty of eternal destruction, away from the presence of the Lord and from the glory of His power.” (2 Thess. 1:9)
Consider three other 'eternal' verses from Hebrews: eternal salvation (Heb. 5:9), eternal judgment (Heb. 6:2) and eternal redemption (Heb. 9:12) it is clear that 'eternal' is an adjective that modifies the result-noun.
So, as we consider 'eternal salvation', we are not being continually saved throughout eternity, but our salvation is eternal - of the age to come and permanent. Concerning 'eternal judgment' Jesus does not continue judging through eternity but His judgment, once handed down, is eternal. And so on. As we read about 'eternal redemption', God is not redeeming His people forever, it is a complete redemption that lasts forever.
And finally as we look at our verse at hand, we consider 'eternal destruction'. God will not be 'destroying' people for eternity (that doesn't even make sense). The penalty of destruction is eternal - of the age to come and permanent. Read Matthew 10:28 where Jesus says, “…fear Him who is able to destroy both soul and body in hell [Gehenna].”
*The Quality of Another Aeon
"While it is unquestionably true that the Bible uses "eternal" to describe God...as also the redeemed...it is clear that the New Testament sometimes uses the word in a qualitative sense... In this view time is divided into two ages - the present age and the age to come (Matt. 12:32, Luke 20:34-35)." See also Eph. 1:21.
"The present age is under Satan's dominion (2 Cor. 4:4), and Christ gave Himself for our sins to rescue us from it (Gal. 1:4). The age to come is of another order that may be called 'eternal'."
"That the age to come is eternal in quality is seen in the fact that the life of the age to come (eternal life) is possible even in the present age through faith in Jesus. (It indicates a life that is different in quality from the life which characterizes the present age.)"
"Based on... Jewish eschatological usage, aionios sometimes suggests 'quality of being, almost meaning 'divine' rather than enduring.' It describes things that are bound to the kingdom of God... The word speaks of 'being of which time is not a measure.'"
Given this definition, 'eternal' punishment and 'eternal' fire are fire and punishment that 'partake of the nature of the aion,' that are 'peculiar to the realm and the nature of God.' The real point is the character of the punishment. It is 'that of the order of the age to come as contrasted with any earthly penalties.' When the New Testament speaks of 'eternal' life... the adjective aionios refers to 'the quality more than to the length of life.'"
"Traditionalist Bruce Milne correctly states: 'The word commonly rendered 'eternal' in our New Testament translations is in fact literally 'of the age (to come).' Thus it refers in the first instance to a particular quality of life, rather than to its durational quantity.'"
"To say that aionios has a qualitative meaning...represents the sober thinking of a cross section of scholars, including several who hold the traditional view of hell." 1
1 Edward Fudge, "The Fire That Consumes" pages 36-37