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

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.