\newtheorem

\newtheorem{env_name}{caption}[within] \newtheorem{env_name}[numbered_like]{caption}

This command defines a theorem-like environment.

env_name The name of the environment -- a string of letters. Must not be the name of an existing environment or counter.
caption The text printed at the beginning of the environment, right before the number.
within The name of an already defined counter, usually of a sectional unit. Provides a means of resetting the new theorem counter within the sectional unit.
numbered_like The name of an already defined theorem-like environment.

The \newtheorem command may have at most one optional argument.