Illustrated Architecture Dictionary
Celtic (Revival) cross
A cross with a long vertical shaft and short horizontal arms (Latin cross), and with a circle superimposed on its center
Celtic: "In art historical terms, works produced by the Celtic peoples, in Western Europe from around 450 BCE to the first century CE, and in the British Isles and Ireland especially during the first millennium CE." -Great Books/Great Art
Such crosses formed a major part of Celtic art. After the 15th century, ringed crosses ceased to be created in the Celtic lands until the Celtic Revival of the 19th century. New versions of the High Cross quickly became fashionable cemetery monuments.
In pagan times, this cross, with axis enclosed by a circle, was a symbol of fertility and life, the cross representing male potency and the circle, female power.
The Celtic cross did not become a common symbol of Christianity until the 4th century. Prevalent in Ireland, it is now primarily a Christian symbol signifying the unity of heaven and earth. Also, the circle is emblematic of God who, like the circle has no beginning and no end.
Often called the Presbyterian Cross, it is the official cross of the Church of Scotland. The design is also referred to as the Irish Cross.
Celtic crosses may have had their origins in the early Coptic (Egyptian) church. There is a similarity between the ankh and the Celtic cross.
The term strapwork is sometimes used to describe the geometric pattterns.
Knotwork History & Symbolism, by Stephen Walker
Celtic knots or Celtic interlace are ornamental patterns that first became associated with Celtic people in the early Celtic Church where they were used to decorate Bible manuscripts, monuments (notably Celtic crosses and cross slabs) and jewelry. They probably were used in other media such as wood carving and textiles but these have not survived.
Knotwork tradition in manuscript painting possibly came to Ireland in the middle of the 7th century in manuscripts illuminated by Coptic monks from Egypt or Syria.
Celtic knots are complete loops with no end or beginning.
Celtic animal interlace is similar in construction but the cords terminate in feet, heads, tails etc. The animal designs are very much influenced by older Saxon and Pictish traditions of abstract beast forms.
The Book of Kells is the best known source of Celtic knots as well as other types of Celtic ornament. The Book of Kells is a fantastic collection of paintings that illuminate the four Gospels in Latin, penned circa 800 AD.
See also: Celtic Spirituality in the Windows of Westminster Presbyterian Church: Earth and Heaven Combined
Examples from Buffalo architecture: