researcher

a research student (i.e. a visiting or non-matriculating graduate student) / a researcher

research association; research society / group of enthusiasts (as may be found in schools, colleges, etc)

research & development; R&D

(education) research project, done by Japanese schoolchildren during summer break, to be presented upon return to school

volunteering, service activity, labor of love

active person / political activist

active (given to action rather than contemplation)

(physiology) action potential

extracurricular activities; club activities / school club