1. (in neurophysiology) The one-way transfer of a nerve impulse from one neuron to another across a synapse. See also neurotransmitter. Compare propagation.

2. (in medicine) The spread of an infection from person to person. This can occur in various ways, such as close contact with an infected person, including sexual contact (see sexually transmitted disease); contact with a vector or a carrier of the disease; consuming food or drink contaminated with the infecting microorganism; and breathing in contaminated droplets of moisture, produced by coughing and sneezing.