A text is cohesive when the elements are tied together and considered meaningful to the reader

Lexical cohesion

Lexical cohesion is the fifth and last type of the cohesive relations in English. It is generally understood as “the cohesive effect [that is] achieved by the selection of vocabulary” (HALLIDAY & HASAN 1994:274). This type of cohesion can be subdivided into the categories reiteration and collocation. Reiteration has to do with the use of general nouns to create a cohesive effect by replacing one element by another in the ongoing text/discourse. Cohesion can thereby be achieved in many different ways, either by the repetition of the same item, or via the use of synonyms, near-synonyms, hyperonyms (superordinates), and general words. It is important to note that “a general noun in a cohesive function is almost always accompanied by the reference item the” which creates anaphoric reference (HALLIDAY & HASAN 1994: 275). The word “the” indicates that the element which comes after it refers back and, therefore, has to be identical with a lexical item that occurred earlier in the text/discourse. Example (9) shows the different types of reiteration that can create lexical cohesion.

Referencing

There are two referential devices that can create cohesion:

Anaphoric reference occurs when the writer refers back to someone or something that has been previously identified, to avoid repetition. Some examples: replacing "the taxi driver" with the pronoun "he" or "two girls" with "they". Another example can be found in formulaic sequences such as "as stated previously" or "the aforementioned".

Cataphoric reference is the opposite of anaphora: a reference forward as opposed to backward in the discourse. Something is introduced in the abstract before it is identified. For example: "Here he comes, our award-winning host... it's John Doe!" Cataphoric references can also be found in written text.
There is one more referential device, which cannot create cohesion:

Exophoric reference is used to describe generics or abstracts without ever identifying them (in contrast to anaphora and cataphora, which do identify the entity and thus are forms of endophora): e.g. rather than introduce a concept, the writer refers to it by a generic word such as "everything". The prefix "exo" means "outside", and the persons or events referred to in this manner are never identified by the writer. Halliday and Hasan considered exophoric reference as not cohesive, since it does not tie two elements together into in text.

A homophoric reference is a generic phrase that obtains a specific meaning through knowledge of its context. For example, the meaning of the phrase "the Queen" may be determined by the country in which it is spoken.

Ellipsis
Ellipsis is another cohesive device. It happens when, after a more specific mention, words are omitted when the phrase must be repeated.

A simple conversational example:

A: Where are you going?

B: To dance.
The full form of B's reply would be: "I am going to dance".

A simple written example: The younger child was very outgoing, the older much more reserved.

The omitted words from the second clause are "child" and "was".

Substitution

A word is not omitted, as in ellipsis, but is substituted for another, more general word. For example, "Which ice-cream would you like?" – "I would like the pink one," where "one" is used instead of repeating "ice-cream."

Lexical Cohesion

Lexical cohesion refers to the way related words are chosen to link elements of a text. There are two forms: repetition and collocation. Repetition uses the same word, or synonyms, antonyms, etc. For example, "Which dress are you going to wear?" – "I will wear my green frock," uses the synonyms "dress" and "frock" for lexical cohesion. Collocation uses related words that typically go together or tend to repeat the same meaning. An example is the phrase "once upon a time". This is lexical cohesion.