Anabasii, in antiquity, were couriers who travelled on horseback or in chariots, quickly bringing messages and commands from elsewhere. The word comes from the Greek αναβασις (adscensus, "mounting").
Anabasii were regularly used by the Greeks and Romans starting at the time of Augustus. The Persians had mounted couriers before this, though; Xenophon attributed the first use of couriers to Cyrus the Younger. Anabasii changed horses regularly, in order to maintain a swift rate of travel.
In Roman Britain, Rufinus made use of anabasii, as documented in Saint Jerome's memoirs (adv. Ruffinum, l. 3. c. 1.): "Idcircone Cereales et Anabasii tui per diversas provincias cucurrerunt, ut laudes meas legerent?" ("Is it on that account that your Cereales and Anabasii circulated through many provinces, so that they might read my praises?")
Other types of couriers contemporary to the anabasii included the Greek hemeredromi, who carried their messages by foot.