Question: *Plesionyms* are synonymous words which have slight differences in meaning.

What are the examples of it? I found:

* Fog v Mist
* Fearless v Brave

When and why are they are used?

What are the aspects which differentiate plesionymic synonyms from cognitive synonyms?

Answer: 
Why would you choose one word over another, when the two might be synonyms?

The best explanation was provided by [Isaac Asimov](https://en.wikipedia.org/wiki/Isaac_Asimov):

> 
> R. Daneel said, "I do not understand the distinction you are making,
> Partner Elijah. Since 'murder' and 'homicide' are both used to
> represent the violent ending of the life of a human being, the two
> words must be interchangeable. Where, then, is the distinction?"
> 
> 
> "Of the two words, one screamed out will more effectively chill the
> blood of a human being than the other will, Daneel."
> 
> 
> "Why is that?"
> 
> 
> "Connotations and associations; the subtle effect, not of dictionary
> meaning, but of years of usage; the nature of the sentences and
> conditions and events in which one has experienced the use of one word
> as compared with that of the other."
> 
> 
> "There is nothing of this in my programming," said Daneel [...].
> 
> 
> *(from ["The Robots of Dawn"](https://en.wikipedia.org/wiki/The_Robots_of_Dawn))*
> 
> 
>
It depends on the context.

In a nearly polar land with frequent precipitation there will be many words used for **snow** and they aren't all synonyms: they describe different types of snow which are relevant to the environment.

But in an equatorial land where it never snows, the words **snow** and **sleet** and **hail** might be considered to be synonyms.