1. in neuroscience, a hypothetical process in which the activation of one neuron is presumed to spread to connected neurons, making it more likely that they will fire.

2. in cognitive psychology, an analogous model for the association of ideas, memories, and the like, based on the notion that activation of one item stored in memory travels through associated links to activate another item. As each item is activated, further activation may spread through the network, making it more likely that associated items will be recalled. Spreading activation is a feature of some network-memory models of semantic memory.