Pseudochirality or pseudoasymmetry occurs when chirality is reliant only on other atoms in the molecule that are chiral. Most commonly, a stereocenter with 2 identical substituents differing only in having opposite chirality are bonded to the same stereocenter. For example, if substituents 1, 2, 3 and 4 are bonded to a single carbon and substituents 3 and 4 are the same only differing in that 3 is R and 4 is S, the molecule would be pseudochiral. This is because if substituents 3 and 4 have the same chirality, there would no longer be 4 different substituents bonded to the center atom which disestablishes the center atom as a stereocenter and renders the molecule unable to be chiral. When an atom is pseudochiral, it's nomenclature is (r/s) instead of (R/S). An important note is that R takes priority over S in assigning CIP priority. In the pictured example the stereocenter C(s) is bonded to Cl, H, C(R) and C(S). Since the two carbons are opposite in terms of R/S, they count as two distinct and differing substituents this makes the stereocenter C(s) pseudochiral. Pseudochirality can affect the effectiveness of some drugs.
Pseudochirality can also exist in the form of axial chirality most commonly in atropisomers, allenes and alkylidene cycloalkanes.