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1.
Performing with a high degree of precision, as in when one "runs a clinic."
Their game is getting clinical these days!
by CaptainMorganSFPL June 28, 2009
 
2.
Something that is a typical example (or a 'textbook' example) of something such as a symptom of an illness (e.g. a runny nose & a high temperature) which often presented to a medical clinic is easily diagnosed. Clinical may be a synonym for 'textbook' or 'textbook case'.
An WOW addict quits playing because he determined it was ruining his life and a few weeks later begins heavily playing some other MMO game - That's Clinical - a clear example of (in this case) an addictive personality who needs help
by Cookie1224 December 09, 2011
 
3.
To be depressed, as in clinical depression.
She is so clinical today, do you think she's okay?
by TaraM May 24, 2005
 
4.
Mihali
"He was clinical on and off the park today"
by Spartanigga III September 07, 2012