What is acid?

definition of acid (Arrhenius acid): It is the compound upon dissolve in water gives H+.

according to lewis (lewis acid): It is substance accept electrons from lewis base.

also defined as substance which is oxidized upon reaction.

also important thing of acids is that upon reaction with base it gives water and salt in reaction called neutralization.

examples:

1- HCl + NaOH ------> NaCl +H2O
    acid     base               salt     water
here HCl lose H+ whic binds to (OH)- giving water then Na binds to Cl giving NaCl

2- H2SO4 + Ca(OH)2 -------> 2 H2O + CaSO4
    acid          base                       water    salt

salt meaning precipitation formed by reaction between acid and base.

according to lewis acid: here H+ (has no electrons) react to (OH)- in which oxygen which has negative charge due to excess electron on it.

negative  oxygen(in hydroxide)  coordinate with 2 electrons for H+ binding it by coordinated bond.