a move in which you roll one direction, then reverse back in anothe rdirection in an attempt to shake off someone on your 6 - use of throttle and/or rudder/flaps in conjunction might cause an overshoot. the best way to counter someone attempting a scissors is to go vertical, bleeding speed and then drop back down on their 6, reacquiring the speed you lost and hoping for a cleaner shot before the guy breaks away.
it could either be vertical or horizontal, with the vertical less common than the horizontal (most commonly found on the deck after you latch onto someone's 6 and they're trying to shake you off.)
the pix on left is a vertical one, the one on right is a horizontal one. this is actually a duel where both went into double immelmanns at the merge (the vertical scissors) that led to loss of e which transited into the horizontal scissors.