The strength of a synapse has been defined by Bernard Katz as the product of (presynaptic) release probability pr, quantal size q (the postsynaptic response to the release of a single neurotransmitter vesicle, a quantum), and n, the number of release sites. "Unitary connection" usually refers to an unknown number of individual synapses connecting a presynaptic neuron to a postsynaptic neuron.  
The amplitude of postsynaptic potentials (PSPs) can be as low as 0.4 mV to as high as 20 mV.  The amplitude of a PSP can be modulated by neuromodulators or can change as a result of previous activity.  Changes in the synaptic strength can be short-term, lasting seconds to minutes, or long-term (long-term potentiation, or LTP), lasting hours.  Learning and memory are believed to result from long-term changes in synaptic strength, via a mechanism known as synaptic plasticity.