A phenomenon, opposite to LTP, which had, however, different mechanics.
LTD is a decrease in response for afferent stimulation when this stumulation is too strong/frequent (and, from the other hand, if this stumulation doesn't induct LTP). The convinient interpretation for LTD is that it's caused by neurotransmitter depletion. LTD is thus considered as an electrophysiological model for gabituation.
In some cases, however, it's shown for more complex pre- and post-synaptic mechanisms to take part in LTD phenomenon (such as receptor modifications, retrograde messengers secretion etc.)