(1):
(adv.) At the same time; simultaneously.
(2):
(adv.) Side by side; also, opposite; over against; on a line with the vessel's beam; - with of.
(3):
(adv.) Up to a certain level or line; equally advanced; as, to keep abreast of [or with] the present state of science.
(4):
(adv.) Side by side, with breasts in a line; as, "Two men could hardly walk abreast."