An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H+). 

On a related note, acid test is a general term for verified, approved, or tested in a large number of fields; however, its origin comes from its use by gold miners in the 1850's. The acid test was used for testing true gold where nitric acid was poured on a gold or a similiar-looking substance. Pure gold would remain chemically inert when in contact with nitric acid, whereas gold's look-alike, iron pyrite (also known as "fool's gold"), would dissolve. 

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