from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The heir apparent to an ancient Celtic chief, elected during the chief's lifetime.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The heir presumptive to a Celtic clan.
from the GNU version of the Collaborative International Dictionary of English
- n. In Ireland, a lord or proprietor of a tract of land or of a castle, elected by a family, under the system of tanistry.
from The Century Dictionary and Cyclopedia
- n. The chief, or holder of the lands and honors, in certain Celtic races; sometimes, the chief's chosen successor. See tanistry.
The new ruler was elected unless a tanist (a lieutenant with right of succession) had been elected already.
After the Gospel the king was made to set his right foot in the foot-print of Fergus Mor Mac Erca, the impression of which was cut in stone; there he took an oath to preserve all the ancient customs of the country and to leave the succession to the tanist.
Kings, though taken from one family, were elective, the tanist or heir-apparent being frequently not the nearest relation of him who reigned.
Lover, for her love he prowled with colonel Richard Burke, tanist of his sept, under the walls of Clerkenwell and, crouching, saw a flame of vengeance hurl them upward in the fog.
To the tanist of O'Donnell, and the Prince of Nial's race.
North Munster mourned the loss of either its chief or its tanist; some great families lost three, five, or seven brothers on that sanguinary day.
 This was also the case with the Irish tanists, or chiefs of septs; the people elected a tanist, but their choice was confined to the members of the ruling family.
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While standing on the stone, the chief took an oath to preserve all the customs of the country, and the rights of the tanist, or territorial heir.