Synchronic (syn = together, chronos = time).
The synchronic approach is concerned with the vocabulary of a language at a given period of time.
Diachronic (dia = through, chronos = time).
The diachronic approach deals with the changes and the development of the vocabulary in the course of time.
Synchronic approach deals with special descriptive lexicology, diachronic approach deals with special historic lexicology.
The two approaches should not be contrasted: they are interconnected and interdependent.
Language is the reality of thought, and thought develops together with the development of society, therefore language and its vocabulary must be studied in the light of social history. Every new phenomenon in human society and in human activity in general finds a reflection in vocabulary.
E.g.: nylon (technology), sputnik (science), perestroika (social and politi-cal life).
A word, through its meaning rendering some notion, is a generalized reflection of reality.