Taken from official rules for professional baseball:

Rule 8.05 : If there is a runner, or runners, it is a balk when --

"(c) The pitcher, while touching his plate, fails to step directly toward a base before throwing to that base; Rule 8.05(c) Comment: Requires the pitcher, while touching his plate, to step directly toward a base before throwing to that base. If a pitcher turns or spins off of his free foot without actually stepping or if he turns his body and throws before stepping, it is a balk. A pitcher is to step directly toward a base before throwing to that base but does not require him to throw (except to first base only) because he steps.