Introductions to masters in relativity: time's an illusion (6)
I believe the answer is:
mirage
'an illusion' is the definition.
(thesaurus)
'introductions to masters in relativity time's' is the wordplay.
'introductions to' suggests taking the first letters.
'time' becomes 'age' (age is a kind of time).
The initial letters of 'masters in relativity' is 'mir'.
'mir'+'age'='MIRAGE'
(Other definitions for mirage that I've seen before include "Something illusory and unattainable" , "Illusion caused by atmospheric refraction" , "Optical illusion, in a desert say" , "You might see this in desert" , "It doesn't exist" .)