1. The ability of an organism to withstand extreme variations in environmental conditions, such as drought.

2. The build-up of resistance to drugs or other chemicals (such as pesticides), which occurs after prolonged use or application. Increasingly large doses of the chemical are required to produce the desired effect in the organism.

3. (immunological tolerance) The phenomenon by which the cells of the immune system are constrained from mounting an immune response against ‘self’ tissues. During their development and maturation, lymphocyte precursors (i.e. precursors of both B cells and T cells) undergo a series of selection processes to ensure that they are capable of recognizing the body’s own tissue markers, particularly the histocompatibility proteins, and that they do not respond to the wide range of other ‘self’ antigens when the latter are combined with these marker proteins. Any precursor lymphocytes that fail these selection procedures are eliminated, ensuring that only tolerant clones are produced (see thymocyte). The term ‘tolerance’ also embraces failure of the immune response in an animal exposed to a foreign antigen to which an immune response would normally be mounted. This commonly follows exposure to the antigen during fetal life, presumably when the immune system is developing self-tolerance.