The direction of the line from apex to base of a prism in a principal section (a section lying in a plane perpendicular to the refracting edge). The setting position for the base of a prism is normally specified by the direction 'base-up' (or base-down, in or out as the case may be) in which 'up' and 'down' have their ordinary meanings, 'in' means towards the nose and 'out' towards the temple. Base-apex line, base-apex meridian and base-apex direction are deprecated terms (British Standard). Alternatively, the TABO notation is used. Abbreviations for the placement of the base of the prism are BD (for base-down), BI (for base towards the nose), BO (for base towards the temple) and BU (for base-up). See standard axis notation; base of prism.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann