standard state
A standard state is an agreed-upon 'home base' for a substance — a fixed, clearly specified condition that everyone uses as a common starting line. Because energy and free energy only ever have meaning as differences (there is no absolute zero of enthalpy or Gibbs energy you could pin down), chemists need a shared reference so that one person's numbers can be added to another's. The standard state is that reference.
By convention, the standard state is the pure substance at a specified pressure of exactly 1 bar (close to ordinary atmospheric pressure), in its most stable form at the temperature of interest — usually quoted at 25 °C. For a solute in solution, the standard state is a concentration of 1 mole per litre behaving ideally. Quantities measured against this baseline carry a little superscript circle, like ΔG° or H°, marking them as 'standard' values.
Why it matters: thanks to standard states, tabulated values such as standard enthalpies of formation and standard Gibbs energies can be combined to predict any reaction, even ones nobody has run. The honest caveat: 'standard' fixes pressure (and concentration), but not temperature — 25 °C is merely the most common choice, so always check which temperature a table assumes.
The standard state of carbon is graphite, not diamond — so the standard enthalpy of formation of graphite is defined as exactly zero, while diamond comes out slightly positive, measured against that graphite home base.
The most stable form at the chosen conditions sets the zero point.
The reference pressure was changed from 1 atm to exactly 1 bar in 1982; the difference is tiny (about 1%) but means very old tables and new ones can disagree slightly. The superscript symbol is properly a plimsoll mark (a circle with a horizontal bar).