verifying what constitutes an "environment variable"

[Robert P. J. Day]

  i just want to make sure i have the technically correct definition
of an environment variable WRT makefiles.

  based on testing, there are four ways i can set an env var that will
automatically be passed to a further make invocation:

  1) part of my linux shell environment
  2) $ VAR=value make ...
  3) $ make VAR=value ...
  4) in the makefile, "export VAR = value"

i realize there are some subtle differences in the above choices but,
basically, do they all fit the definition of an "environment
variable"?  thanks.

rday