Definition

Thick:

-Adjective

• Relatively great in extent from one surface to the opposite in its smallest solid dimension.
• Measuring a certain number of units in this dimension.
• Heavy in build; thickset. 
• Densely crowded or packed. 
• Having a viscous consistency. 
• Abounding in number. 
• Impenetrable to sight. 
• Difficult to understand, or poorly articulated. 
• (informal) Stupid.
• (informal) Friendly or intimate. 
• Deep, intense, or profound.

 -Adverb

• In a thick manner.
• Thickly. 

-Noun

• The thickest, or most active or intense part of something.

Thin:

-Adjective

• Slim, narrow in size, and (of a person or animal) usually carrying little fat.
• Of low viscosity or low specific gravity, e.g., as is water compared to honey.
• Scarce.
• Overly strict.
• (sports) Describing a poorly played golf shot where the ball is struck by the bottom part of the club head. 

-Verb

to thin (third-person singular simple present thins, present participle thinning, simple past and past participle thinned)

• (transitive) To make thin or thinner.
• (intransitive) To become thin or thinner.
• To dilute.
• To remove some plants in order to improve the growth of those remaining.

-Noun

• Thin, in philately, a loss or tearing of paper from the back of a stamp, although not sufficient to create a complete hole.