Semasiology studies the change in meaning which words undergo. The meaning of a word can change in the course of the historical development of language. Changes of lexical meanings can be proved by comparing contexts of different times. Transfer of the meaning is called a lexico-semantic word-building. In such cases the outer aspect of a word does not change. The causes of semantic changes can be extra-linguistic and linguistic.

By extra-linguistic causes we mean various changes in the life of the speech community, changes in economic and social structure, scientifi c concepts and other spheres of human activities as refl ected in word meaning. For example, the change of the lexical meaning of the noun «pen» was due to extra-linguistic causes. Primarily «pen» comes back to the Latin word «penna» (a feather of a bird). As people wrote with goose pens the name was transferred to steel pens which were later on used for writing. Still later any instrument for writing was called «a pen». On the other hand causes can be linguistic it means factors acting within the language system. The main form of linguistic cause is discrimination/differentiation of synonyms which can be illustrated by the semantic development of a number of words. The confl ict of synonyms when a perfect synonym of a native word is borrowed from some other language one of them may specialize in its meaning, e.g. in Old English the noun «tide» was polysemantic and denoted «time», «season», «hour». When the French words «time», «season», «hour» were borrowed into English they ousted the word «tide» in these meanings. It was specialized and now means (regular movement of sea towards and away from the land). Thus, the vocabulary is the most fl exible part of the language and it is precisely its semantic aspect that responds most readily to every change in the human activity in whatever sphere it may happen to take place.