1. (in neurophysiology) The effect of successive stimuli on the postsynaptic membrane, which results in the generation of an excitatory postsynaptic potential (EPSP). Although a single impulse may fail to cross the junction between adjacent nerve cells, the synapse becomes more responsive to the following impulse.

2. (in ecology) The phenomenon observed during succession in which the presence of one species increases the likelihood or speed of colonization by a second species. The first species brings about some change in the environment that make it more favourable for the second. For example, a pre-existing plant may provide the germinating seeds and seedlings of another species with vital shelter from the wind or protection from herbivores. Alternatively, it may alter the nature of the soil, for example by changing the pH, sufficiently to permit growth of a new incoming species. See also foundation species.