An ELT Glossary : Collocation

Definition : The tendency of words to co-occur regularly and form "partnerships" in ways not fully explained by semantic compatibility (or lack of it).

Examples :  you can have a "bad cold" or a "heavy cold". You can have a "bad headache" but not a "heavy headache"; hard  and heavily both collocate with the verb rain (it was raining hard/heavily) bot only heavy collocates with the noun rain (The forecast is for heavy rain, not *hard rain).