A collocation is a combination of two or more words that frequently appear together. You can say heavy rain but not strong rain. You can make a decision but not do a decision. There is often no logical rule — the words simply go together by convention and must be learned as whole chunks.
Collocations cover every grammatical type: verb + noun, adjective + noun, adverb + adjective, adverb + verb, noun + noun, and fixed phrases. They also vary in strength (from weak to fixed) and register (formal, neutral, informal).
Not all collocations are equally tight. Fixed collocations cannot be changed at all. Strong collocations have a small, predictable set of partners. Weak collocations are more flexible — one word pairs naturally with a wide range of partners. Click any segment to see examples.
Fixed collocations are so tightly bound that no word can be substituted. The meaning is often not predictable from the individual words — many are idioms. Even changing one small word breaks them completely.
Strong collocations have a small, predictable group of partners. You can sometimes substitute a word and still be understood, but a native speaker would always choose the established partner.
Weak collocations are made up of words that combine freely with a wide range of partners. "Weak" does not mean unimportant — it means the word is flexible. Native speakers still have preferred combinations.
Register describes how appropriate a collocation is for a given context — formal (F), neutral (N), or informal (I). The same meaning can often be expressed at all three levels, but using the wrong one sounds jarring.