What are the differences between tonicity and osmolarity?

Basis of differentiation

Tonicity

Osmolarity

Definition

Is the measurement of the osmotic pressure difference between two solutions separated by a semipermeable membrane assessed by comparing their water potentials

The concentration of all osmotically active particles in a solution, expressed as the number of osmoles (or milliosmoles) of solute per liter of solution

Types of solutions

Isotonic, hypertonic, and hypotonic solutions

Hypersomotic, isosmotic, and hypoosmotic solutions 

Type of solute

It quantifies the concentration of solutes that cannot cross a semipermeable barrier

It measures the combined concentration of solutes that can and cannot pass through a semipermeable membrane

Relative to

Relative to the solute concentration inside a cell

Is an absolute measure of solute concentration and independent of any specific cellular environment