\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.