下面是英语词汇学系列讲座，育路教育网特别为您搜集整理，内容如下：
Chapter 7
Changes in Word Meaning
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