There are two questions- Why are we "St. Peter’s Episcopal" in "St. Mary’s Parish" and what is a parish, historic and current ?
1. In our history there are mentions of St. Peter’s being part of St. Mary’s Parish, formed in 1677. Thus, St. Mary’s Parish was founded long before St. Peter’s Episcopal (1735).
St. Mary’s Parish originally included an area on both sides of the Rappahannock River, In 1713, that part of St. Mary’s Parish lying in Richmond Co was added to another parish. When Caroline Co was formed from Essex Co (also King William & King and Queen Co), St. Mary’s Parish went too.
So what is a Parish ? A parish in colonial Virginia was a unit of both civil and religious authority that covered a set geographical territory. Usually covering much less territory than a county, the parish was the layer of government closest to the people, and for many it probably had a greater day-to-day impact on their lives than the county or colony-wide government.
Parishes were given authority over "all matters concerning the vestry, their agreements with their ministers, touching the church-wardens, the poore and other things concerninge the parish or parishoners respectively be referred to their owne ordering and disposeing from time to time as they shall think fitt."
As territorial divisions, the parishes’ bounds were set by Virginia’s General Assembly. They also divided parishes, often at the request of parishioner
Gradually after the Revolutionary War powers that were attached to the Parishes were taken over by local government even before the Episcopal Church was set up.
The historical word has given way to a word connected with the canons and theology.
2. The parish is an entity under the current canons and historical (theological) entity
From the website "Anglican Pastor"
"The term “parish” comes from the paroikos, meaning “near the household.” The bishop in the ancient Church, as opposed to occupying an office, occupied a household (oikos) in which members of the Church gathered in a common life. This was how clergy were trained, service to the poor was rendered, and how the lonely were brought into community. In this, the bishop maintained his status as that of the Latin pater familias, a head of a household of 50 or so people. As the need for more local communities of Christians arose, the bishops placed clergy from the household in charge of these local congregations. Their nearness to the household maintained a bond to the bishop, and through the bishop, to the whole Church.
Saint Cyprian says of this: the episcopate (the body of bishops) is one; it is the whole in which each enjoys full possession. The Church is likewise one, though she be spread abroad, and multiplies with the increase of her progeny: even as the sun has rays many and one light.”
"The Church mirrors the dual natures of Christ – both human and divine – in that the church is a body both human and divine, a divine humanity in which the Holy Spirit dwells. Thus, the Church is both visible and invisible, made up of human members, but at the same time – the body of Christ. The church shows forth the Gospel in the world because the church is not purely an “idea,” but a living body, increasing not by individual conversion, but by the establishment of local churches in her communion, what we should readily call “parishes.”