What is the following activity known as in vocabulary learning? Find words which can befriend the word 'rain'. e.g. heavy rain.

Collocation refers to words that naturally and habitually 'go together' or 'befriend' each other in a language — like 'heavy rain', 'strong tea', 'make a decision', 'fast food'. Asking learners to find words that befriend a given word (like 'rain' — heavy, light, torrential, pouring) is teaching collocation. This is distinct from general phrases, word groups, or word webs which are broader concepts.