From DT Online
In engineering, Stress is a measure of the load applied for each unit of cross-sectional area of the material under load - i.e. Stress = Load / Area and is measured in newtons per square metre (Nm-2).
When a material is subjected to stress it will deform in some way (e.g. be stretched or squashed) - although for many materials this may be only by a microscopically small amount. Even a seemingly solid material is made of atoms and molecules which are held together by chemical bonds. This can be modelled with beads and springs as shown. As the material is stressed the bonds become deformed.
Many terms in engineering, science and mathematics are represented by Greek symbols and Stress is allocated the symbol σ (sigma).