Electron deficiency occurs when a compound has too few valence electrons for the connections between atoms to be described as covalent bonds. Electron deficient bonds are often better described as 3-center-2-electron bonds. Examples of compounds that are electron deficient are the boranes.
The term electron-deficient is also used in a more general way in organic chemistry, to indicate a pi-system such as an alkene or arene that has electron-withdrawing groups attached, as found in nitrobenzene or acrylonitrile. Instead of showing the nucleated character common with simple C=C bonds, electron-deficient pi-systems may be electrophilic and susceptible to nucleophilic attack, as is seen in the Michael addition or in nucleophilic aromatic substitution.
Electron deficiency can also be used to describe molecules with highly polarized bonds such as boron trifluoride or silicon tetrafluoride which have a strong tendency to act as Lewis acids.
As the most extreme form of electron deficiency one can consider the metallic bond.