The phrase "single ion" refers to a single charged particle. It means that there is only one atom or molecule that has gained or lost an electric charge. Full definition
A team of researchers at Columbia Engineering has used miniaturized electronics to measure the activity of individual ion - channel proteins with temporal resolution as fine as one microsecond, producing the fastest recordings of single ion channels ever performed. (sciencedaily.com)
The growth of these clusters is then considered to take place by the addition of single ions, and the formation of different polymorphs is considered to be under thermodynamic or kinetic control. (science.sciencemag.org)
Experimental realization, by using 9Be + as the logic ion and 27Al + as the spectroscopy ion, indicates the feasibility of applying this technique to make accurate optical clocks based on single ions. (science.sciencemag.org)