Coalesce ((v) : to come together as one, to fuse, to unite

Coerce(v) to force someone to do or not to do something

He tried to flatter, bribe and even coerce, but he was never able to make her reveal the hidden rebel base. 

Cogent (adj) powerfully convincing

He was cogent is explaining why he needed the confidential files, so we gave them to him.

Cognitive (adj) dealing with how we know the world around us through our senses, mental

Scientists study the cognitive apparatus of human beings to pattern how computers should gather information about the world.

Cognizant (adj) aware, conscious

To be cognizant of your responsibilities is to know what your responsibilities are.

 

Coherent (adj) holding together, making sense

A coherent wad of cotton balls is one that holds together.

Cohere: hold together

Colloquial (adj) conversational, informal in language

‘The way I figure it’ is a colloquial expression, or a colloquialism; people often say it, but it isn’t used in formal prose.

Collusion (n) conspiracy, secret cooperation

The increase in petroleum prices was the result of collusion by the oil-producing nations.

Commensurate (adj) equal, proportionate

The number of touchdowns scored by the team and the number of its victories were commensurate (both zero).

Compelling (adj) forceful, causing to yield

A compelling argument for buying a security system is one that makes you go out and buy a security system.

Compendium (n) a summary, an abridgment

A yearbook often contains a compendium of the offenses, achievements, and future plans of the members of the senior class.