Lexical Cohesion

Lexical cohesive devices involve one lexical item entering a relationship with another. A key notion of lexical cohesion is the concept of semantic field  or general area of meaning. Specific topics and concepts are associated in the mind of the user of language with particular groups of words. So, texts belonging to a particular area of meaning  draw on  a range of words specifically related to that area of meaning.
As Halliday and Hasan illustrate, it is not enough for words to have similar meanings to be considered as coming from the same semantic field. If we take pair of words with similar meaning we can soon arrive to a very peculiar collection of terms:

Lion and cat are two felines;
Cat and dog are two domestic animals;
Dog and chicken are two farmyard animals;
Chicken and sausage are  two examples of food;
But LION and SAUSAGE can't go together, they don't belong to the same semantic field.

Words that belong to the same semantic field must have some sense relationship, or logical link, between them.
In Halliday and Hasan view, relationships that qualify word association as part of the same semantic field are of two main kinds: reiteration and collocation.
Reiteration is divided into five types: repetition, synonymy, hyponymy-hyperonymy, meronymy and antonymy.
Repetition
Synonymy
Hyponymy-Hyperonymy: involving the inclusion of one term (a Hyponym) in a superordinate category (a hypernym) --> furniture and table, table is an hyponym of furniture, so furniture is an hypernym of table.
Meronymy: involving a part-whole relationship
Antonymy: associates words in term of opposites

Collocation, the second type of lexical cohesion, involves the relationship between words on the basis of the fact that these often occur in the same surrounding: you should know a word by the company it keeps.

Grammatical and lexical cohesion work in tandem to create textuality. Each element entering a grammatical or lexical cohesive relation contributes to create a cohesive chain. The text gain unity:
a)the more components enter a chain
b)the greater the number of chains
c)the closer the chains intertwine and overlap (si sovrappongono e si intrecciano).