Inhibitory postsynaptic potential
A type of postsynaptic potential where the binding of neurotransmitters with the postsynaptic receptors causes the opening of ion channels which results in hyperpolarization (or the net gain of negative charge across the membrane) thus the firing of an action potential from the postsynaptic cell is less likely.
In general, the resting potential of a neuron is about -70mV and the firing threshold of an action potential is about -50mV. When there is a net gain of negative charge across the membrane of a postsynaptic neuron, the potential moves further to zero or away from the firing threshold. Hence, the postsynaptic potential is inhibitory since the probability to fire an action potential is reduced.
Compare: excitatory postsynaptic potential