halidom comes from Old English hāligdōm ('holiness, sanctuary, holy relic'), from hālig ('holy') + -dōm (the suffix forming nouns of state or domain). Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the noun sense is "a holy place or sacred thing, often invoked in solemn oaths", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as holy, hallow, and hallowed. Its semantic neighborhood includes sanctuary, sanctity, and holiness, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English hāligdōm and Old English hālig.