A material is brittle if it is liable to fracture when subjected to stress. That is, it has little tendency to deform (or strain) before fracture. This fracture absorbs relatively little energy, even in materials of high strength, and usually makes a snapping sound.
When used in materials science, it is generally applied to materials that fail in tension rather than shear, or when there is little or no evidence of plastic deformation before failure.
When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturally malleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducing grain size, dispersion strengthening, work hardening, etc.), but if this is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving material toughness is therefore a balancing act.
This principle generalizes to other classes of material. Naturally brittle materials, such as ceramics (most famously glass), are difficult to toughen effectively. Most such techniques involve one of two mechanisms: to deflect the tip of a propagating crack, for instance by introducing natural weaknesses of limited extent, or to create carefully controlled residual stresses so that cracks from certain predictable sources will be forced closed, as in the case of toughened glass and pre-stressed concrete. Both mechanisms tend to soften the material somewhat, although most ceramics are quite hard to begin with. The least-brittle structural ceramics are silicon carbide (mainly by virtue of its high strength) and transformation-toughened zirconia.
Generally, the brittle strength of a material can be increased by pressure. This happens as an example in the brittle-ductile transition zone at an approximate depth of 10 km in the Earth's crust, at which rock becomes less likely to fracture, and more likely to deform ductilely.
Supersonic fracture is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart (Markus J. Buehler and Huajian Gao) and IBM Almaden Research Center in San Jose, California (Farid F. Abraham).