In biology and biochemistry, osmosis is about the movement of free water molecules across a semi permeable membrane.  Osmosis can also be viewed more generally as about the movement of solvent particles across a semi permeable membrane. 

Osmosis can be defined as the movement(diffusion) of free water molecules across a semipermeable membrane from a higher concentration of free water molecules to a region of lower concentration of free water molecules.  A semipermeable membrane is a membrane that allows only molecules smaller than water to pass through. 

Side A contains a higher concentration of free water molecules than side B. On side B many free water molecules have become locked by sodium ions. These locked up water molecules are too big and can't get through the semi permeable membrane.

The volume in side B will increase until there is an equal number of free water molecules on each side. This increase in volume is also known as osmotic pressure.  image: ClearlyExplained.Com

The concept of osmosis  is  typically described as the movement of solvents(water) across a semi permeable membrane from a lower concentration of solute to higher solute concentration. 

"Osmosis is the movement of solvent (typically water) from the region of lower to higher solute concentration."

Although osmosis is traditionally considered the movement of water, the principles can apply to any other kinds of molecules, including gases, with a similar set up as shown in the above diagram.