The spike bayonet is a bayonet where the blade is shaped like a spike and not like a knife. A bayonet of this type is only suitable for stabbing and not for cutting or slashing. Many early socket bayonets was spike bayonets. Today the spike bayonet is not much seen in use, and with the exception of China this kind of bayonets has not been much used after WWII.
One of the best know spike bayonet in modern time, is the British Lee-Enfield No4 spike bayonet. This bayonet was cheap and easy to make and was widely used in WWII.