peer
A unit of communications hardware or software that is on the
same {protocol layer} of a network as another. A common way
of viewing a communications link is as two {protocol stack}s,
which are actually connected only at the very lowest
(physical) layer, but can be regarded as being connected at
each higher layer by virtue of the services provided by the
lower layers. Peer-to-peer communication refers to these real
or virtual connections between corresponding systems in each
layer.
To give a simple example, when two people talk to each other,
the lowest layer is the physical layer which concerns the
sound pressure waves travelling from mouth to ear (so mouths
and ears are peers) the next layer might be the speech and
hearing centres in the people's brains and the top layer their
cerebellums or minds. Although, barring telepathy, nothing
passes directly between the two minds, there is a peer-to-peer
communication between them.