System of signs

Semantic changes.
Causes of semantic changes:

Extra-linguistic causes:
1) historical – changes in the way of life
2) psychological – taboos of different kind. F.e. “bathroom” is a euphemism for “toilet”.
Linguistic causes:
1) ellipsis – takes place when in a phrase made up of two words one of them is emitted and its meaning is transferred to its partner. F.e.: “to starve”. In OE it meant “to die”. In the 16^th century the verb developed a new meaning – “to die of hunger”.
2) differentiation of synonyms. F.e. “land”. In OE the word “land” had two meanings: “solid part of the surface of earth” and “the territory of the nation“. Later the word “country” was borrowed and the word “land” lost the second meaning due to the competition of synonyms.
3) fixed context is cause closely connected with the competition of synonyms. F.e. “heaven”. In OE it meant “sky”. Then the word “sky” was borrowed and “heaven” became restricted in use (religion, poetic spheres).

References:

1. 
   Beynon-Davies P. (2010). "Dances with bees: exploring the relevance of the study of animal communication to informatics". International Journal of Information Management. 30 (1): 185–198. doi:10.1016/j.ijinfomgt.2010.02.001.
   
2. 
   https://cseweb.ucsd.edu/
   
3. 
   https://www.cambridge.org/