What is an acid

An acid is a substance that donates protons (in the Brønsted-Lowry definition) or accepts a pair of valence electrons to form a bond (in the Lewis definition).

According to Arrhenius theory an acid is any substance that yields hydronium ins on disassociating in water. According to Brownsted-Lowry theory acids are proton donors

An acid is a substance that is corrosive in nature, tastes sour and turns red litmus blue.