Accumulation / Distribution (A/D) is one of forex technical indicator used in technical analysis. This indicator is the accumulation of the distinction between all upward movements. This indicator used to help determine if the market controlled by buyers (accumulation) or by sellers (distribution).

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From Investopedia
A momentum indicator that attempts to measure the demand by determining whether the investors are generally “accumulate” (purchase) or “distribution” (sale) of a particular share by identifying differences between share price and volume flow. It is calculated as follows:
Acc/Dist = ((Close – Low) – (High – Close)) / (High – Low) * Period’s volume
(full article)

From Metaquotes
Accumulation / Distribution Technical indicator is the changes in price and volume. The volume acts as a weighting to the change in the price - the higher the coefficient (the volume), the greater is the contribution of price change (this time) on the value of the indicator. (full article)

From Wikipedia
Accumulation / Distribution Index is a cumulative total volume indicator of the technical analysis by Marc Chaikin, added or subtracted each day’s volume in relation to which lies between the close of today’s High and Low. (full article)