Osmolarity vs. Tonicity
You're correct that tonicity needs two solutions to define.

Osmolarity (or osmotic concentration) is the measure of solute concentration, defined as the number of osmoles of solute per litre (L) of solution (Osm/L).

Tonicity, on the other hand, refers to the relative concentration of two solutions separated by a semipermeable membrane. 

The difference is based what is considered for osmosis and tonicity. In case of osmosis, it is the water (or solvent) which moves across the membrane, while tonicity depends on the solutes which cannot move across the membrane. From Wikipedia:


  ... osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes, whereas tonicity takes into account the total concentration of only non-penetrating solutes.