‘. . . . until a certain period of time.’
Jude uses αιονιος in writing of the vengeance of eternal fire in v7, and of ‘the mercy of our Lord Jesus Christ unto eternal life, v21. But in writing of everlasting chains (I am staying with the use of eternal and everlasting as used in the KJV, for now) he uses this word αιδιοις, its only other use in the New Testament scripture (the Stephens 1550 text) being Romans 1:20 regarding the nature of God, himself, his eternal power.
Strong (126) states eternal, so also does Young and I had to revert to my one thousand page Liddell & Scott, American Edition, 1854, to get any further. There I find that the word is either derived from, or equivalent to, αειδελος which is linked to, or derived from, αειδης.
These two words convey meanings of ‘unseen’, ‘without form’ and ‘immaterial’.
I notice that the AV, Young's Literal and JND link αιονιος only to 'power' in Romans 1:20. The EGNT, however, correctly renders the Greek word order and states :
. . both eternal his power and divinity . . . .
η τε αιδιος αυτου δυναμις και θειοτης
It appears to me that αιδιοις emphasises, in Romans, the immaterial, invisible perpetuity of him who is spirit and, therefore, when applied, in Jude, to the chains binding spirit-beings - who have no bodily manifestation - that their state of captivity, which will have no end, is being emphasised as formless, insubstantial and invisible. This state is also attributed to souls, who will experience the ‘blackness of darkness for ever’ having lived as Jude describes in v8 to v16.
But Jude also describes the eternal fire of those men of Sodom and Gomorrha, which fire is also described, as a lake of fire by John in Revelation 19:20 and which is stated to eventually contain Diabolos and his demonic followers.
Both states, of angels who kept not their first state, and of men, who turned the grace of God into lasciviousness, are described by words which emphasise the bleak isolation of their deserved plight; a state which does not change, and continues - eternally - to be punctuated, but not altered, by the judicial process of judgment.
Prisoners on remand, who are clearly and unequivocally guilty, are captive before, during and after their trial. So also, I understand, are angels and men who have rebelled against the word of God and so made themselves unworthy of an existence of any other kind than scripture here describes.