Coherence measures the degree of semantic similarity between the words in topics generated by a topic model. The more similar the words within a topic are, the higher the coherence score, and hence the better the topic model. There are a number of ways to calculate coherence based on different methods for grouping words for comparison, calculating probabilities of word co-occurrences, and aggregating them into a final coherence measure. Coherence is a popular approach for quantitatively evaluating topic models and has good implementations in coding languages such as Python and Java.