Branches of linguistics. Synchronic vs diachronic approaches to the language study. Lexicology – ‘the science of the word’

An archaism (Gr. archáios ‘ancient’) is a word that was once common but is now replaced by a synonym; it remains in the language, but mostly belong to the poetic style and are used for creating a stylistic effect, e.g.
betwixt, prep. ‘between’;
A historism is a word which denotes a thing that is no longer used; unlike archaisms, they are not replaced by synonyms. Historisms are very numerous as names for social relations, institutions, objects of material culture of the past, e.g. transport means:
brougham /'bru:(ə)m/, n. ‘a horse-drawn carriage with a roof, four wheels, and an open driver's seat in front ’;
A neologism (Gr néos ‘new’ and logos ‘word, study’) is a new lexical unit introduced into a language to denote a new object or phenomenon. The term is first attested in English in 1772, borrowed from French néologisme. Neologisms are often directly attributable to a specific person, publication, period, or event.
In January 2002 Collins Gem English Pocket Dictionary editorial board have registered 140 neologisms.
Collins Essential English Dictionary (2003) contains 5,500 new words.
The Oxford Dictionary of New Words (1999) includes articles on 2,000 new words and phrases prominent in the media or public eye in the 80s -90s.
While the typical lexical growth areas of the 1980s were the media, computers, finance, money, environment, political correctness, youth culture and music, the 1990s saw significant lexical expansion in the areas of politics, the media and the Internet.
Nonce words (occasional words) (an ellipsis of the phrase for the nonce ‘for the once’) are lexical units created by the speaker on the spur of the moment, for a given occasion only, and may be considered as ‘potentially’ existing in the English vocabulary, e.g. what-d’you-call-him /-her/-it/-them, n. is used instead of a name that one cannot remember.
A lot of neologisms resulted from nonce words, e.g. yuppie, n. ‘a well-paid young middle-class professional who works in a city job and has a luxurious lifestyle’; coach potato, soap opera, generation X, thirty-something, glass ceiling ‘an unacknowledged barrier to advancement in a profession, especially affecting women and members of minorities’; gerrymander /'dʒɛrɪ‚mandə/, v. ‘manipulate the boundaries of (an electoral constituency) so as to favour one party or class’.