Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. This can be observed as Long-Term Potentiation (LTP) or Long-Term Depression (LTD). LTP is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them simultaneously. LTD, on the other hand, is a long-term decrease in synaptic strength.