A binary language belongs to $\mathrm{RP}$ if a polynomial-time probabilistic machine never accepts a nonmember and accepts each member with probability at least $2/3$. It belongs to $\mathrm{coRP}$ if its complement belongs to $\mathrm{RP}$.

The success threshold $2/3$ is a standard constant-error convention for $\mathrm{RP}$; it gives the same class as the customary threshold $1/2$. The complement is taken among binary words, not among languages.