[OT] What is the "imaginary mass" in a Tachyonic field?

[Georgi Guninski]

This confuses me:
https://en.wikipedia.org/w/index.php?title=Tachyonic_field&oldid=730595004
| A tachyonic field, or simply tachyon, is a quantum field with an imaginary mass.

"imaginary" links to imaginary number, using i^2= -1.


So far, as I read it, this means the mass can be say 1i
grams.

Later comes the confusion, wiki explains "imaginary" as real 
(in the math sense).

| The "imaginary mass" really means that the system becomes unstable.

IIRC relativity has formulas with factors of the form
sqrt(1-v^2/c^2) which go imaginary when v>c and without
doubt scientists know this.

Someone knows what "imaginary" means in this context?

AFAICT Tachyonic fields exist unconditionally.