It derives from the Latin terminus, and in early medieval Ireland it referred to the sacred boundary land surrounding a church or monastic site, a zone of sanctuary where violence was forbidden and fugitives could seek protection. A place recorded as An Tearmann in County Mayo, then, is not simply a townland with an old-fashioned name; it is likely the trace of an ecclesiastical territory whose original church or foundation has long since vanished or been absorbed into the landscape.

These termon lands were a recognised feature of the early Irish church, and their boundaries were often fiercely defended by the religious community that held them. The abbot or bishop of a monastery could extend protection across the termon, and in some cases the zone stretched for a considerable distance around the sacred centre. Over centuries of Viking raids, Norman reorganisation, and later plantation, many of these ecclesiastical territories lost their formal status, but the name clung to the land. In Connacht particularly, where the pre-Norman church retained influence longer than in some other provinces, termon placenames are a quiet reminder of a religious geography that once shaped daily life in ways that have largely been forgotten.