Definition of grammar:

1- Etymology : The word grammar is derived from Greek « grammatik? techn?» which means « art of letters »

Source: https://en.oxforddictionaries.com/definition/grammar

2- Terminology: The whole system and structure of a language or of languages in general, usually taken as consisting of syntax and morphology (including inflections) and sometimes also phonology and semantics.

Source: https://en.oxforddictionaries.com/definition/grammar

Levels of Language: