In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals exactly). This pressure was changed from 1 atm (101.325 kilopascals) by IUPAC in 1990. The standard state of a material can be defined at any given temperature, most commonly 25 degrees Celsius, although quite a few texts (especially in related disciplines such as physics and engineering) use 0 degrees Celsius for Standard Temperature and Pressure (STP).
It should be noted that the standard state is an arbitrarily chosen non-zero value, not a natural zero point.
For a given material or substance, the standard state is the reference state for the material's thermodynamic state properties such as enthalpy, entropy, Gibbs free energy, and for many other material standards. The standard enthalpy of formation for an element in its standard state is 0.
When the standard state is referred to a solute in a solution, or to a chemical reaction, it also includes the condition that the concentrations of all solutes are at unity (or another designated quantity) for whatever measure of concentration is specified. If that is molarity that would be 1 mol·dm-3 and for molality 1 mol·kg-1 assuming the solution infinite-dilution behaviour. If mole fraction is used, the pure liquid or solid is the standard state (x=1). As it is possible (and in principle legitimate) to take a different unit for each of the species in the reaction, the nature of the standard state needs to be specified when reporting or tabulating. Although a definition involving 1 mol/L (molarity at unity) of A in combination with pure B (mole fraction at unity) is clearly a condition that can never be met, such a non-existent standard state nevertheless leads to a consistent system of tabulated values, provided it is used consistently by all. Of course these values are different from those where a different standard state is adopted.
In chemistry of solutions at elevated temperatures and pressures, the term "standard state" often denotes the hypothetical standard concentration, typically 1 mol/kg for solutes assuming an ideal behaviour (i.e., an infinite dilution), or a unity molar fraction (for pure substances). It does not imply any particular temperature or pressure because, although contrary to IUPAC recommendation, it is more convenient when describing solutions over a wide temperature and pressure ranges.
In the time of their development (the 19th century) the Plimsoll - symbol was adopted as a superscript o to indicate the non-zero nature of the chosen reference state. For typographic reasons this symbol is often abridged to a rather misleading zero superscript 0 in later texts.