Why do they call doctors’ work their practice?

What does Practicus mean? Or is it a straight defination - i.e. it means Doctor's work?

Main Entry: 2practice Pronunciation Guide
Variant(s): also practise \"\
Function: noun
Inflected Form(s): -s
1 a obsolete (1) : performance or operation of something : EXECUTION (2) : a mode of acting or proceeding b : actual performance or application of knowledge as distinguished from mere possession of knowledge : performance or application habitually engaged in; usually : repeated or customary action : USAGE <the practice of rising early or working hard> c (1) : the usual mode or method of doing something <the practice is to use a local anesthetic> (2) practices plural : habitual conduct that is socially, ethically, or otherwise unacceptable <the unwholesome practices of folk medicine> <departing these evil practices>
2 a (1) obsolete : skillful or artful management : dexterity in contrivance or the use of means (2) : treacherous contriving; also : SCHEME, PLOT b archaic (1) : NEGOTIATION (2) : INTRIGUE c archaic : the act of using artifice or influence upon : imposing or working upon
3 a : systematic exercise for instruction or discipline <troops called out for practice> <practice makes perfect> <daily piano practice> b archaic (1) : a practical treatise (2) : practical proficiency or skill acquired by systematic action or exercise <this disease is beyond my practice -- Shakespeare>
4 a : the exercise of a profession or occupation <the practice of law> b : professional business or work especially as an incorporeal property <a lucrative practice> <sold his practice>
5 : the form, manner, and order of conducting and carrying on suits and prosecutions through their various stages according to law
synonym see HABIT
- in practice adverb 1 a : in actual or accepted usage : as a fact b obsolete : in customary or present use : in vogue 2 : in such a condition as a result of practice as to be able to perform in an effective or superior manner <athletes must keep in practice>
- out of practice adverb : in such a condition as a result of lack of practice as to be unable to perform in an effective or superior way <out of practice on the piano>