A solution is a homogeneous mixture of two or more substances. One of the substances is called a solvent (a substance in which other substance or substances are dissolved). The substances dissolved in a solvent are called solutes.
A solution can exist in a solid, liquid or gas form depending on mixed substances and external conditions such as temperature and pressure.
Isotonic, hypertonic, hypotonic solutions
Terms isotonic, hypertonic and hypotonic are usually used when a reference to cell of a living organism is made, but it can be generally used to compare concentration of a solute in two solutions.
Isotonic solutions are two solutions that have the same concentration of a solute.
Hypertonic solution is one of two solutions that has a higher concentration of a solute.
Hypotonic solution is one of two solutions that has a lower concentration of a solute.