An ion is an atom that has gained or lost one or more electrons and has thereby acquired a net electric charge. A negatively-charged ion is an anion; a positively-charged one is a cation.
Baristatic filters cause negatively-charged ion pollution. (TNG: "True Q")
The Chamra Vortex included ionized gas which interfered with sensors. (DS9: "Vortex")
Ion distribution must be checked when analyzing subspace infrared telemetry. (VOY: "Good Shepherd")