microstructure
Microstructure is the arrangement of features you can see under a microscope - the grains, phases, and boundaries inside a solid - at scales from roughly a tenth of a micrometer up to about a millimeter. To the naked eye a piece of steel looks like a uniform gray solid; polish and etch it and put it under a microscope, and a hidden landscape of tiny crystal patches (grains) appears. That landscape is the microstructure.
Most engineering solids are polycrystalline: they are made of many small crystals called grains, each with the same atomic pattern but tilted in a different direction, meeting at grain boundaries (the mismatched seams where two differently-oriented crystal patches meet, like tiles laid at different angles). The microstructure also records which phases are present (regions of different composition or crystal structure), how big the grains are, and their shape. Metallographers reveal it by grinding, polishing, and etching a sample, then viewing it with an optical or electron microscope.
Microstructure is the crucial middle link in the structure-property chain: processing sets the microstructure, and the microstructure sets many mechanical properties. Smaller grains make a metal stronger (the Hall-Petch effect); the fineness of the phases in steel decides its hardness. Because the same alloy can have wildly different microstructures depending on heat treatment, two identical-looking parts can behave completely differently.
Two steel bars of identical composition: one cooled slowly shows coarse, soft grains under the microscope; the other quenched shows a fine, needle-like structure (martensite). Same chemistry, different microstructure - and the quenched bar is several times harder.
What the microscope reveals explains what the material does.
Microstructure is not fixed by chemistry - it is set by processing and can be changed afterward by heat treatment. That is precisely why 'what alloy is it?' is not enough; you must also ask 'how was it processed?'