The words coherence and cohesion both come from the Latin verb cohere which means “to hold together.”
But what does this have to do with writing?
When we talk about coherence in writing (as writers), we talk about how the piece of writing moves through ideas; how the sentences and paragraphs seamlessly flow into one another; how there are no noticeable transitions because each idea naturally leads into the next.
Readers perceive coherence as the sentences creating a complete picture, a whole. Coherence is important for that very reason. Writers want to communicate a message to a reader, and in order to communicate that message effectively, it must come across as a unified idea. Jolts and shifts in the progression of ideas fragment the message and distract the reader.
As mentioned before, coherence and cohesion both come from the same Latin verb, but when those words are used to talk about writing, they convey different meanings. Cohesion is considered the flow of a piece of writing: the grammatical ties that connect each sentence with what has gone before. For example, pronouns refer back to their antecedents. Cohesion is the ease of sentence-to-sentence reading on the grammatical level. Coherence is the logical and natural progression of ideas from sentence to sentence, and paragraph to paragraph. It is a judge of how well the parts create a whole.