What is an acid?

According to lewis,
“Any specie which can accept an electron pair from a donor compound is called an acid.”
Hydrogen,copper,iron,etc are examples of lewis acid.
Other properties of an acid:
¤Acid is proton donor.
¤Acid when dissolve in water gives proton
¤Acids are sour in taste.
¤Acid can change the colour of litmus paper from blue to red.
¤Acids are used in the neutralization reaction (the reaction between an acid and base and the product is salt and water ).
¤Any specie having PH less than 7 is considered as an acid.