1.Types of changes
1)Extension
The extension of meaning, the opposite of restriction, means the widening of a word’s sense until it covers much more than what it originally conveyed.
2)Narrowing
Narrowing of meaning, also called restriction of meaning, means that a word of wide meaning acquires a narrower, specialised sense which is applicable to only one of the objects it previously denoted.
3)Elevation
When the meaning of a word narrows toward a more favourable meaning it is called elevation
4)Degradation
Degradation means the falling of word meaning into disrepute, for one reason or another. Words once respectable or neutral may shift to a less respectable, or even derogatory meaning.
2. Causes of changes
1)Extra-linguistic factors
(1)historical reason
(2)class reason
(3)psychological reason
2)Linguistic factors
(1)shortening
(2)borrowing
(3)analogy