\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 to be defined. A string of letters. It 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. This may simply say "Theorem", for example.

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.