I found the entry from the Princeton Dictionary remarkable. Its long, but I will quote it here in full.
We could say, its all about destroying vikalpa, couldn't we.vikalpa. (P. vikappa; T. rnam par rtog pa; C. fenbie; J. fun betsu; K. punbyol 分別). In Sanskrit, "[false] discrimination," "imagining,” or “conception”, the discriminative activities of mind, generally portrayed in the negative sense of fantasy and imagination, and often equivalent to "conceptual proliferation" (PRAPANCA). Vikalpa refers to the conceptual activities of the mental consciousness (MANOVIJNANA), a mediated mental activity that operates through the medium of generic images (SAMANYALAKSANA). Vikalpa is often opposed to the immediate knowledge provided by direct perception (PRATYAKSA). The direct perception of reality is therefore commonly described as NIRVIKALPA, or “free from thought." 'if Three types of conceptual discrimination (TRIVIKALPA) are typically described in the literature. (1) Intrinsic discrimination (SVABHAVAVIKALPA), which refers to the initial advertence of thought (VITARKA) and the subsequent sustained attention (VICARA) to a perceived object of the six sensory consciousnesses (VIJNANA), that is, the discrimination of present objects, as when visual consciousness perceives a visual object. (2) Conceptualizing discrimination (ABHINIRUPANAVIKALPA), which refers to discursive thought on ideas that arise in the sixth mental consciousness when it adverts toward a mental object that is associated with any of the three time periods of past, present, or future. (3) Discrimination involving reflection on past events (ANUSMARANAVIKALPA), which refers to discriminative thought involving the memory of past objects. 'if There is a wide range of opinion as to the value of vikalpa (in the sense of "thought" or "conception") in the soteriological progress. Some traditions would hold that the structured use of conceptual and logical analysis (and especially the use of inference, or ANUMANA) is a prerequisite to reaching a state beyond all thought. Such a position is advocated in the Indian philosophical schools and in those that favor the so-called gradual path to enlightenment. In the stages of the path to enlightenment, all forms of meditation prior to the attainment of the path of vision (DARSANAMARGA) are "conceptual" and thus entail vikalpa. Other schools radically d value all thought as an obstacle to the understanding of the ultimate and would claim that the nonconceptual, described in some cases as "no-thought" (C. WUNIAN), is accessible at all times. Such an approach, most famously expounded in the CHAN traditions of Asia, is associated with the so-called sudden path to enlightenment (see DUNWU). 'if In the YOGACARA school, vikalpa is described specifically as the "discriminative conception of apprehended and apprehender" (GRAHYAGRAHAKAVIKALPA), referring to the misconception that there is an inherent bifurcation between a perceiving subject (grahaka) and its perceived objects (grahya). This bifurcation occurs because of false imagining (ABHUTAPARIKALPA), the tendency of the relative phenomena (PARATANTRA) to be misperceived as divided into a perceiving self and a perceived object that is external to it. By relying on these false imaginings to construct our sense of what is real, we inevitably subject ourselves to continued suffering (DUHKHA) within the cycle of birth-and-death (SAMSARA). Overcoming this bifurcation leads to the nondiscriminative wisdom (NIRVIKALPAJNANA), which, in the five-stage path (PANCAMARGA) system, marks the inception of the path of vision (darsanamarga), where the adept sees reality directly, without the intercession of concepts. The elimination of grahyagrahakavikalpa proceeds from the less to the more subtle. It is easier to realize that a projected object is a projection than to realize that a projecting subject is as well; among projected objects, it is easier to realize that afflicted (SAMKLISTA) dharmas (the SKANDHAs and so on) are projections than to realize that purified (VYAVADANA) dharmas (the five paths and so on) are as well; and among subjects it is easier to realize that a material subject (a mental substratum and so on) is a projection than to realize that a nominally existing subject (a nominally existing self and so on) is. This explanation of vikalpa, common in the PRAJNAPARAMITA commentarial tradition, influenced the theory of the SAMPANNAKRAMA (completion stage) in ANUTTARAYOGA (highest yoga) TANTRA, where prior to reaching enlightenment the four sets of vikalpas are dissolved with their associated PRANAS in the central channel (AVADHOTI).
Vaibhsikas identify three vikalpas - these are listed above as svabhavavikalpa, abhinirupanavikalpa, and anusmaranavikalpa. The five sense consciousnesses only have the first kind of vipalka which is the same as vitarka.
Vasubandhu goes on to explain that basically all mental activity that is not dhyana is vikalpa.