thicken

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive & intransitive verb To make or become thick or thicker.
• transitive & intransitive verb To make or become more intense, intricate, or complex.

from The Century Dictionary.

• To become thick or thicker.
• To become deeper or heavier; gain bulk.
• Of a liquid, to approach more nearly a state of solidity; gain firmer consistency; also, to become turbid or cloudy.
• To become dark or obscure; specifically, of the weather, etc., to become misty or foggy.
• To grow more intense, profound, animated, intricate, etc.; become complicated.
• To gain in number or frequency; hence, to crowd; throng.
• To become indistinct.
• To make thick or thicker.
• To increase in depth, or distance between opposite surfaces; hence, figuratively, to make stouter or more substantial; strengthen.
• Of liquids, to increase the consistency of; inspissate: as, to thicken gravy with flour; also, to render turbid or cloudy.
• To obscure with clouds or mist; befog.
• To make more numerous or frequent; redouble: as, to thicken blows.
• noun A spelling of thick 'un (which see, under thick, a.).

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To become thick.
• transitive verb To make thick (in any sense of the word).
• transitive verb To render dense; to inspissate.
• transitive verb To make close; to fill up interstices in.
• transitive verb obsolete To strengthen; to confirm.
• transitive verb To make more frequent.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb transitive To make thicker (in the sense of wider).
• verb transitive To make thicker (in the sense of more viscous)
• verb intransitive To become thicker (in the sense of wider).
• verb intransitive To become thicker (in the sense of more viscous).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb make viscous or dense
• verb make thick or thicker
• verb become thick or thicker