Definition of "practitioner"

The [Isaac] Newton that emerges from the [unpublished] manuscripts is far from the popular image of a rational practitioner of cold and pure reason. The architect of modern science was himself not very modern. He was obsessed with alchemy.

2014 June 21, “Magician’s brain”, in The Economist, volume 411, number 8892

[…] the men of St. John's were cunning practitioners, in shaking off their Masters and Heads.

c. 1572, John Whitgift, Admonition to the Parliament