How is Microenvironment strictly defined?
The meaning of the term microenvironment depends on the scale of the system that you're looking at. It's a sort of weasel word that loosely means "context" or "stuff in the immediate surroundings of the system of interest that has some kind of an effect on it". I've seen it used to describe everything from the chemical microenvironment of a cell (e.g. a hormone gradient) to the electrostatic microenvironment of a single amino acid residue in a protein (e.g. the field from nearby charged residues).

Conceptually it's similar to the "surroundings" in the "system and surroundings" framework from statistical physics.