The Collaborative International Dictionary of English v.0.48:
Acid \Ac"id\, n.
1. A sour substance.
2. (Chem.) One of a class of compounds, generally but not
always distinguished by their sour taste, solubility in
water, and reddening of vegetable blue or violet colors.
They are also characterized by the power of destroying the
distinctive properties of alkalies or bases, combining
with them to form salts, at the same time losing their own
peculiar properties. They all contain hydrogen, united
with a more negative element or radical, either alone, or
more generally with oxygen, and take their names from this
negative element or radical. Those which contain no oxygen
are sometimes called hydracids in distinction from the
others which are called oxygen acids or oxacids.
Note: In certain cases, sulphur, selenium, or tellurium may
take the place of oxygen, and the corresponding
compounds are called respectively sulphur acids or
sulphacids, selenium acids, or tellurium acids.
When the hydrogen of an acid is replaced by a positive
element or radical, a salt is formed, and hence acids
are sometimes named as salts of hydrogen; as hydrogen
nitrate for nitric acid, hydrogen sulphate for
sulphuric acid, etc. In the old chemistry the name acid
was applied to the oxides of the negative or
nonmetallic elements, now sometimes called anhydrides.