The pericallosal artery is the distal segment of the anterior cerebral artery that extends from the exit of the callosomarginal artery along the sulcus of corpus callosum to the splenium.
For some authors (Rhoton et al. for example) and in angiographic practice, this definition is not clear because the callosomarginal artery is not always present and variable, so the variablity of origin of the callosomarginal artery could place the origin of the pericallosal artery at any point from near the anterior communicating artery. Thus these authors use another definition for the pericallosal artery: the pericallosal artery is the segment of the anterior cerebral artery extending from the exit of the anterior communicating artery along the segments of the corpus callosum to the splenium. In this definition, the pericallosal artery begin with the A2 segment and is equivalent to the term "distal ACA".
(Microsurgical anatomy of the distal anterior cerebral artery. Perlmutter D, Rhoton AL Jr. - J Neurosurg. 1978 Aug;49(2):204-28.)