macrostate
/ MAK-roh-stayt /
Imagine describing a jar of marbles to a friend over the phone. You will not list where every marble sits — you will say 'about half red, half blue, the jar is full.' That short summary is a macrostate: the few big, measurable facts about a system, like its temperature, pressure, volume and how much stuff it contains.
More precisely, a macrostate is the description of a system in terms of its bulk thermodynamic variables, the ones you can read off an instrument. A single macrostate can be realised by an enormous number of microscopic arrangements (microstates) — every detailed shuffling of the molecules that still gives the same temperature, pressure and energy. The macrostate ignores all that hidden detail and keeps only the summary.
Why it matters: the whole game of statistical mechanics is counting how many microstates belong to a given macrostate, because that count is what entropy measures. A key caveat: two systems can look identical at the macrostate level while being wildly different molecule by molecule, and that hidden multiplicity is exactly what drives the direction of spontaneous change.
Toss four coins and report only 'two heads, two tails.' That is one macrostate — but it can happen six different ways (HHTT, HTHT, and so on), while 'four heads' has only one way. The lopsided macrostate is favoured simply because it owns more microstates.
One macrostate ('two heads, two tails') is built from many microstates.
Do not confuse macrostate with microstate: the macrostate is the coarse summary you measure, the microstate is one fully detailed arrangement. Entropy is the bridge — it counts the microstates inside a macrostate.