tropic: either of two parallels of latitude about 23.5 degrees to the north and south of the equator representing the points farthest north and south at which the sun can shine directly overhead and constituting the boundaries of the Torrid Zone or tropics
entity: that which is perceived or known or inferred to have its own distinct existence (living or nonliving)
tickler coil: a small coil in series with the anode of a vacuum tube and coupled to the grid to provide feedback