The term voltage is commonly used as a short name for voltage difference. In this introduction, the term "voltage" is used to mean a specific value (not a difference), and is denoted by the symbol V. The SI unit for voltage is the volt (symbol: V). Note that (in International Standards and in printed material that conforms to the International Standards) "sloping vee" (V) and "upright vee" (V) have the related but different meanings just stated.

The voltage difference between two (electron) positions "A" and "B", inside a solid electrical conductor (or inside two separate, electrically-connected, solid electrical conductors), is denoted by (V_A − V_B). This voltage difference (V_A − V_B) is the electrical driving force that drives a conventional electric current in the direction A to B. Voltage difference can be directly measured by an "ideal voltmeter". Well-constructed, correctly used, real voltmeters approximate very well to ideal voltmeters. For non-scientists, an analogy involving the flow of water is sometimes helpful in understanding the concept of voltage difference (see below).

Precise modern and historic definitions of voltage difference exist, but (due to the development of the electron theory of metal conduction in the period 1897 to 1933, and to developments in theoretical surface science from about 1910 to about 1950, particularly the theory of local work function) some older definitions are not now regarded as strictly correct. This is because they neglect the existence of "chemical" effects and surface effects. A particular lesson from surface science is that, to get consistency and universality, formal definitions must relate to positions or (better) electron states inside conductors. A modern form of definition follows.

It is now universally accepted that, in conduction processes as they occur in metals and most other solids, electric currents consist exclusively (or almost exclusively) of the flow of electrons in the direction B to A. This movement of electrons is controlled by differences in a so-called "total local thermodynamic potential" often denoted by the symbol µ ("mu"). This parameter µ is often called the "local Fermi level", but is sometimes called the "(local) electrochemical potential of an electron" or the "total (local) chemical potential of an electron". The modern electron-based definition of voltage difference (V_A − V_B) is in terms of differences in µ:

$(V\_\{\backslash mathrm\{A\}\}\; -\; V\_\{\backslash mathrm\{B\}\})\; =\; \backslash ;\; -(\{\backslash mu\}\_\{\backslash mathrm\{A\}\}\; -\; \{\backslash mu\}\_\{\backslash mathrm\{B\}\})/e$ ,

where e is the elementary positive charge. It is sometimes convenient to put µ_B=0 and V_B=0, and choose position "B" so that it can be a convenient reference zero for V. It is common to choose position "B" to be inside a good electrical conductor solidly connected (by a very-low-electrical-resistance path) to the local "Earth" or "Ground". In the analysis of electrical circuit diagrams, it is common to show the point in the circuit that is being taken as the reference position B, by attaching a "Ground" ("Earth") symbol to this point.