Metonymy

Metonymy is a figure of speech in which one thing is used to refer to another thing that is closely associated with it.

For example, the phrase "the White House" is often used to refer to the president of the United States, even though the White House is actually the building where the president lives and works.

Another example, when we  say that a farmer has three hands working for him. In this case, “hands”  refers to laborers – i.e. people who use their hands  at work.

Metaphor

Metaphors can be used to create a more vivid image in the reader's mind, or to make a connection between two things that might not seem to be related.
Like metonymy, metaphor is a means of using language figuratively, which can be conventionalized to create new cases of polysemy. Metaphor involves seeing similarities between different things and describing one as if it were the other. 
For example: John is  a rat. Also, we can see an example of conventionalized metaphor in another sense of hand: as in the hands of a clock. Here the metaphor plays on the similarities between the hands of people and the pointers on a clock.

Broadening and Narrowing

Another way in which words can have different senses is if they are auto hyponyms. That is, if one of the word senses is a more specific version of another of its senses. This can happen through the broadening or narrowing of one of the word senses.

An example of an auto hyponym is "paper": 

Broad sense:"paper" refers to any material manufactured in thin sheets from the pulp of wood or other fibrous substances, used for writing, printing, or wrapping.
Specific sense: "paper" can also refer specifically to a piece of written or printed material, such as a document or newspaper.
Here, "paper" is an auto hyponym because it encompasses both the general material and the specific items made from that material.

Narrowing

Usually, the verb “drink” means ‘consume liquid by mouth’ or ‘consume alcohol by mouth,’ as in examples (1) and (2) below, respectively.

for example: After surgery, Jen could only drink through a straw. ( Broad: water, juice, soup etc.)

After his liver transplant, George swore never to drink again. ( Narrow: drink alcohol)

In this case, the ‘consume alcohol’ sense in (2) is a subcategory of the ‘consume liquid’ sense in (1) – the original ‘liquid’ meaning has been narrowed.

Broadening

An example of broadening is Yankee, which in its original meaning denotes specifically people from the northern United States (in contrast to those from the South), but now it can also denote someone from any part of the US, in contrast to those from other countries.

Conversion

Finally, words can also take on new senses by changing their grammatical category. 

For example, from noun to verb, or from content word to function word. If a word keeps the same form (that is, it doesn’t have a prefix or suffix and keeps the same pronunciation) when it changes from one category to another, then it has undergone a process known as conversion (or zero derivation). 

For example, verbs expressing emotional or mental states can often be used as nouns for those states:

• Love (N)= Love (V)
• Hope (N)= hope (V)
• Fear ( N)= fear (V)

In the noun-to-verb direction, all of the following names for liquids can also be verbs

• Paint (N)= paint (V)
• Oil (N)= oil (V)
• Water(N)= water (V)
• Milk ( N)= milk(V)

Have you noticed that one of these is not like the others? While the verbs paint, oil, and water all have senses relating to putting the liquid on/in something, the verb sense for milk involves taking the liquid out of something (an animal). In these cases, conversion from noun to verb has added some meaning, and we can see patterns in how lexical items in one category change when they are converted to other categories.

Generally, conversions can happen from any lexical word class to another. To sum up metonymy, metaphor, broadening and narrowing, and conversion are all important tools that writers and speakers can use to make their language clearer and impressive. These figures of speech can be used to create new images and ideas in the reader's mind, or to make connections between things that might seem to be unrelated. Please refer to my previous article on lexical relations.

References

•    Murphy M. L. ( 2012)  lexical Meaning, Cambridge University Press

•    Hurford, J., B. Heasley and M. Smith (2007) Semantics: a coursebook (2nd edition) Cambridge University Press

•    Bard (google.com)