Vertical position or vertical location is a position along a vertical direction above or below a given vertical datum (reference level). Vertical distance or vertical separation is the distance between two vertical positions. Many vertical coordinates exist for expressing vertical position: depth, height, altitude, elevation, etc. Points lying on an equigeopotential surface are said to be on the same vertical level, as in a water level.
The International Organization for Standardization (ISO), more specifically ISO 19111, offers the following two definitions: 
ISO 6709 (2008 version) makes the following additional definition:
The International Civil Aviation Organization (ICAO) offers similar definitions: 
ICAO further defines:
I.e., elevation would be the altitude of the ground or a building.
Several physical quantities may be defined based on the definitions above:
Vertical distance quantities, such as orthometric height, may be expressed in various units: metres, feet, etc.
Certain vertical coordinates are not based on length, for example, geopotential numbers have units of m2/s2. Normalization by a constant nominal gravity value (units of m/s2) yields units of metre, as in geopotential height (based on standard gravity) or dynamic height (based on normal gravity at 45 degrees latitude). Despite the dimension, the vertical coordinate still does not represent distance, as would be measured with a ruler. Sometimes a standard geopotential metre (symbol gpm or m') is introduced for emphasis.  However, this practice is not acceptable with the International System of Units (SI): the Guide for the Use of the International System of Units, section 7.5 (Unacceptability of mixing information with units), states that "When one gives the value of a quantity, any information concerning the quantity or its conditions of measurement must be presented in such a way as not to be associated with the unit.". 
Another non-SI unit is the vertical metre, introduced when there may be confusion between vertical, horizontal, or slant distances. It is used for distance climbed during sports such as mountaineering, skiing, hiking, running or cycling  In German-speaking countries the abbreviation 'Hm' for Höhenmeter ("height metre") is used; if it is preceded by a '±' it refers to the cumulative elevation gain.
Various instruments and techniques may be used for measuring vertical position:
Many physical phenomena are related to vertical position, as driven by gravity: