Cohesion is the lexical linking within a written or spoken text or sentence that holds a text together and gives it meaning. It gives it unity. It is linked to the broader concept of coherence.
Let’s imagine you write a well-constructed paragraph that all makes sense, one sentence leading into the next. Afterwards, generally, you cannot move the sentences around willy-nilly unless you alter them in some way.
It’s the same, generally, with a completed and well-constructed sentence. You can’t often move words around in a sentence willy-nilly either without making some other modifications.
This is because of cohesion. Five fundamental linguistic mechanisms give writing and speech their cohesion or unified structure:
1. Reference: The boy wanted to surprise her. The next day, he … (he refers back to the boy)
2. Ellipsis: A: Who wrote the letter? B: Bianca. (The response Bianca elliptically signals that Bianca wrote the letter.) Ellipsis is when we leave out of a word or words that are redundant or able to be understood from contextual clues.
3. Substitution: I aim to run a marathon next year. If I do, . . . (do substitutes to run a marathon)
4. Conjunction: Mike needed to lose some weight. He, therefore, decided to buy an exercise bike. (therefore shows the causal relationship between the first and second sentences)
5. Lexical cohesion: (here through synonymy): He was grateful for the money he had been given. He put the cash in his wallet and drove to the shop. (Cash refers back to money.)
We do this naturally, but it can be very challenging for L2 learners, mainly when they are listening to conversations.