A tachyon is a hypothetical faster-than-light particle.

Tachyon Matter
Rolling Tachyon
Tachyon Dynamics in Open String Theory
Tachyon Condensation on the Brane Antibrane System
Universality of the tachyon potential
On Exact Tachyon Potential in Open String Field Theory
Tachyon couplings on non-BPS D-branes and Dirac–Born–Infeld action
Cosmological evolution of the rolling tachyon
Some exact results on tachyon condensation in string field theory
Analytic solution for tachyon condensation in open string field theory