period

n.一段时间，时期；学时，课时；周期，句点

perforce, perform, performance, performer, performing arts, perfume, perfumed, perfumer, perfumery, perfunctorily, perfunctoriness, perfunctory, pergola, perhaps, peri-, perigee, perihelion, peril, perilous, perimeter, period piece, periodic, periodic table, periodical, periodicity, peripatetic, peripatetically, peripheral, peripherally, periphery, periphrasis, periphrastic, periphrastically, periscope, periscopic, perish, perishable, perishables, perished, perisher,