\newenvironment

\newenvironment{nam}[args]{begdef}{enddef} \renewenvironment{nam}[args]{begdef}{enddef}

These commands define or redefine an environment.

Changes in LaTeX2e

It is possible to have an optional argument, the form of the command being
  \newenvironment{nam}[args][opt]{begdef}{enddef}
  \renewenvironment{nam}[args][opt]{begdef}{enddef}
If opt is present, the first of the args arguments is optional with default value opt. If it is absent all arguments are mandatory.
See also Defining commands, Environments
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Revised: Sheldon Green, 16 Nov 1995.