Latin, saisitor.

An installer was a royal official responsible for delivering possession - seisin - of a grant, normally by presenting the writ or charter in one of the public courts of Hundred or shire.

There are three occurrences of the word in Domesday, all in the Claims section for Huntingdonshire and all referring to the misdeeds of Eustace the sheriff. No named individual is described as an installer, which may suggest that the office was a lowly one, perhaps equivalent to the king's messengers of the following and later centuries.

For the royal messenger service, see Mary C. Hill, The king's messengers, 1199-1377 (1961).