A Celtic cross is a symbol that combines a cross with a ring surrounding the intersection. The symbol has ancient origins and was likely a regional variation of the "sun cross" (which has arms that do not extend outside the circle). When Christianity spread to the British Isles, the Celtic cross was combined with the Christian cross. As a result, Christian high crosses (which are made of stone and richly decorated) are often, though not always, built in this design.
It is widely accepted that the Celtic cross has ancient, pre-Christian origins. It is similar to the so-called "sun cross", which can be found in Bronze Age Europe (Nordic Bronze Age, Urnfield culture). The archaic English word for cross as an instrument of torture is rood (literally "pole", cognate with rod). The word cross in English derives only indirectly from Latin crux via Old Irish and possibly Old Norse, introduced in the 10th century.
Celtic crosses may have had origins in the early Coptic church. The similarity between the ankh, symbol of "life" and variations of the cross or ankh with a circle on Coptic stella and textiles from as early as the 5th century clearly show that the combination of circle and cross were used in early Christian Egypt. Although some experts say that the crosses were originally carved horizontally on stone, their geometrical ring construction and the fact that the lights in east-facing high crosses can be seen to refract early morning sunlight is indicative of vertical construction.