what is an acid?

An acid can be defined as a chemical substance whose aqueous solutions are sour taste and turn blue litmus red.Acids react with bases and certain metals (like calcium) to form salts.Aqueous solutions of acids have a pH of less than 7.
Generally there are three common definitions for acids:--
1).Arrhenius definition- acids are those substances which increase the concentration of hydrogen ions (H+),or hydronium ions (H3O+) when dissolved in water.
2).Bronsted-Lowry definition:--Acording to this,acid is a substance which can act as a proton donor i.e any compound which can easily be deprotonated can be considered an acid.For ex- alcohols and amines which contain O-H or N-H fragments.
3).Lewis definition--A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond.For ex-all metal cations,and electron-deficient molecules such as boron trifluoride(BF3) and aluminium trichloride(AlCl3).