A colloid is a mixture of a liquid and a suspension of solid particles so small in size that they are distributed evenly. The mixture can remain liquid or become a paste or a gel.
Many substances of interest in the fields of industry, practical applications or food are in this form, such as adhesives (the word "colloid" comes from the Greek κο ́λλα, glue), inks, cement, jellies and thick compositions such as mayonnaise. Organic substances such as starch or protein solutions easily become colloids.
The behaviour of a colloid is mainly determined by the forces between the particles. They may be attractive or repulsive and are strongly dependent on the physical-chemical conditions (temperature, pressure, acidity etc.).