What is a bulge? Three different definitions have been used so far, based on morphology, photometry, or kinematics, respectively. According to the morphological definition, a bulge is the component of a disc galaxy that swells out of the central part of a disc viewed edge-on. Based on photometry, a bulge is the extra light in the central part of the galaxy, over and above the exponential profile fitting the remaining (non central) part of the disc. The third definition is based on kinematics, and in particular on the value of V / , or, more specifically, on the location of the object on the (V / , ellipticity) diagram (often referred to as the Binney diagram (Binney 1978, 2005)). These three definitions are compared and discussed in Athanassoula & Martinez-Valpuesta (2007).
The lack of a single, clear-cut definition of a bulge, although historically understandable, has led to considerable confusion and to the fact that bulges are an inhomogeneous class of objects. For this reason, Kormendy (1993; see also Kormendy & Kennicutt 2004, hereafter KK04) distinguished classical bulges from pseudo-bulges. However, pseudo-bulges by themselves are also an inhomogeneous class of objects, as argued by Athanassoula (2005a, hereafter A05), who distinguishes three types of objects which are, according to the above definitions, classified as bulges. Classical bulges are formed by gravitational collapse or hierarchical merging of smaller objects and corresponding dissipative gas processes. Their morphological, photometrical and kinematical properties are similar to those of ellipticals. They are discussed extensively in other papers in these proceedings and are not the subject of this review. The two other types of bulges are boxy/peanut bulges (B/P), and discy bulges, which will be discussed here. As stressed in A05, different types of bulges often co-exist and it is possible to find all three types of bulges in the same simulation, or in the same galaxy.