Make a liquid or mixture more viscous or dense

• Often by simmering or adding a thickener

Become more viscous or dense, as a liquid or sauce

Make something physically thicker, wider, or more bulky

• Apply or build up more material, as with hair or coatings

Become denser, more opaque, or more crowded

• Said of fog, hair, traffic, or a crowd

Make or become more complex, detailed, or intense

• Strengthen a case or evidence; complicate a situation