Acquired immunity is of two kinds: active immunity arises when the body produces antibodies against an invading foreign substances (antigen), either through infection or immunization; passive immunity is induced by injection of serum taken from an individual already immune to a particular antigen. Active immunity tends to be long-lasting; passive immunity has a short life.

Immunological tolerance.  The failure to respond to a potential antigen.

Impedance. Any resistance to the flow of fluids moved by a series of pluses, such as blood flow.

Impressed. Sunk below surface level.

Imprinting.
Development in an animal during an early, sensitive period, of the tendency to follow or otherwise approach an object (commonly a parent).

Inbreeding depression. A reduction inviability resulting from increased homozygosity  through Inbreeding.

Incisor. Chisel-shaped tooth in front mouth of mammals Primitively three on each side of upper and lower jaws. Gnawing teeth on rodents which continually grow in length and tusks of elephants are modified incisors.

Inclusion. A particle or structure contained within a cell or organ.

Incompatibility.
(1) In flowering plants, failure to ser seed (i.e. of fertilization and subsequent development of embryo) after pollination (self or cross0 has taken place. Due to inability of pollen tubes to grow down style. In physiologically heterothallic fungi, failure to reproduce sexually in single or paired cultures.
In both cases the incompatibility is genetically determined-it is due to the possession by the individual plant or thalli of genes which prevent the fusion of likes; and the resultant is the same, namely, to promote outerbreeding.
(2) In horticulture, inability of scion to make a successful union with stock.