Speculator, derived from specula, a prospect that is to say a view from the summit of a place, whence anything may be seen advantageously at a distance.
Thus a cohort of this description (Speculatorum Cohors) was established by M. Anthony, that they, from an elevated part of his ships, might explore and act as sentries or watchmen. There were other acceptations of the word, such as spies and even executioners.