Definition
Excitation
Excitation is a synaptic signal that increases the likelihood of a neuron firing.
Biologically, excitation depolarises the soma — it brings the membrane potential closer to the firing threshold. Excitation is mediated by synapses and stands in opposition to inhibition.
McCulloch–Pitts Model
In the McCulloch–Pitts model, excitatory inputs contribute positively to the sum; if the sum reaches the threshold the neuron fires.