IEEE Std 260.4-2018 pdf download – Letter Symbols and Abbreviations for Quantities Used in Acoustics.
3. General principles 3.1 Letter symbols for quantities, unit symbols, and abbreviations Letter symbols include symbols for physical quantities (quantity symbols) and symbols for units in which these quantities are measured (unit symbols). “Letter symbol” as a technical term does not have the same meaning as either “name” or “abbreviation.” A letter symbol represents a physical quantity or a unit and is therefore independent of language. An abbreviation is a letter or a combination of letters (sometimes including an apostrophe or a full stop, i.e., a period) that by convention represents a word or a name in a particular language; hence, an abbreviation is likely to be different in a different language. For example, the letter symbol for electromotive force is E, whereas the abbreviation is “emf” in English, “fem” in French, and “EMK” in German. Additionally, the unit names “ampere” and “second” have sometimes been abbreviated amp and sec, respectively, but this usage is now deprecated. The standard unit symbols for ampere and second are A and s, respectively. The term “physical quantity” as used in this standard means a measurable attribute of phenomena or matter. Some examples are length, mass, and time. A quantity symbol is generally—but not always—a single letter specified as to style of type font (e.g., bold, italic) and modified when appropriate by one or more subscripts or superscripts. For example, the quantity symbol for electric current is I and the quantity symbol for Reynolds number is Re. In a given work, the same letter symbol shall appear consistently throughout for the same physical quantity regardless of the units employed or of special values assigned.
A unit symbol is a letter or group of letters (for example, mm for millimeter), or in a few cases a special sign, that is commonly used in place of the name of a unit. Unit symbols have sometimes been treated in the same manner as general abbreviations, but the recommendations of the International Committee for Weights and Measures (CIPM) and of many other international and national bodies concerned with standardization emphasize the symbolic character of these designations and rigidly prescribe the manner in which they shall be treated. This concept of the unit symbol is therefore adopted in this standard. The use of either the unit symbol or the full name of the unit itself is always permitted.