An attack vector is used for determining if a enemy model is able to be attacked. An attack vector is a line drawn from the centre of a square, marked by a +, to the center of another square. It cannot cross a blocking feature or blocking model.
For close combat it has a range of 1 square, i.e. adjacent squares. For ranged attacks the attack vector can be drawn over any distance.
Sometimes 'line of sight' (LoS) is used as a synonym for an attack vector. It is worth noting that some attacks, abilities and traits may enable an attack vector to be drawn that passes through blocking features and thus out of LoS.