American Heritage Dictionary

noun

1. A tight curl, twist or bend in a length of this material, as one caused by the tensing of a looped section of wire.
2. A painful muscle spasm, as in the neck or back; a crick
3. A difficulty or flaw that is likely to impede operation, as in a plan or system.
4. A mental peculiarity; a quirk.
5. An unusual eccentric idea.

Random House Dictionary

noun

1. A twist or curl, as in a thread, rope, wire or hair, caused by its doubling or bending upon itself.
2. A muscular stiffness or soreness, as in the neck or back..
3. A flaw or imperfection likely to hinder the successful operation of something, as a machine or plan: There are still a few kinks to be worked out of the plan before we start production.
4. A mental twist; notion; whim or crotchet.
5. To form, or cause to form, a kink or kinks, as a rope.

Merriam-Webster’s Medical Dictionary

noun

1. A short tight twist or curl caused by a doubling or winding of something upon itself.
2. A cramp in some part of the body.

WordNet 3.0

noun

1. A painful muscle spasm especially in the neck or back (‘rick’ and ‘wrick’ are British) /syn: crick/
2. A sharp bend in a line produced when a line having a loop is pulled tight.
3. A difficulty or flaw in a plan or operation: “there are still a few kinks to iron out”.