Imaginary time is generally described as a mathematical concept of time which is required to complete the theory of everything (a theory that attempts to unify Einstein’s theory of relativity with that of quantum physics).
Similar to imaginary numbers, imaginary time is created by rotating real time in the complex plane.
Interestingly, when imaginary numbers are used in Feynman’s equations, the difference between space and time completely disappears – imaginary time is indistinguishable from directions in space.ˡ
Despite what the name suggests, imaginary time is NOT imaginary or made up, but in fact it be argued that it is the true time of a finite universe, and what we would call ‘real’ time is just something that’s more practical and also easier for us to digest.
Thanks to imaginary time, the universe can now be described as being finite in imaginary time, but without boundaries or singularities; this has huge consequences not only for quantum physics, but also philosophically.
ˡ combined paraphrased quote from Stephen Hawking - A brief history of time