hornwort: any aquatic plant of the genus Ceratophyllum; forms submerged masses in ponds and slow-flowing streams
aquatic plant: a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth
limiter: (electronics) a nonlinear electronic circuit whose output is limited in amplitude; used to limit the instantaneous amplitude of a waveform (to clip off the peaks of a waveform)