A Top is the practitioner who takes on the role of giving the type of kink play being explored. (1)
Topping is the physical act of giving the kink play. (2)
References:
Easton, D and Hardy, J (2003) The New Book on Topping. Greenery Press. ISBN 9781890159368
Top (2016) kinkley.com. https://www.kinkly.com/definition/707/top-bdsm