As Jerome observes the Greek skandalon may be rendered offense, downfall, or a stumbling against something. For when a body, while moving along a path, meets with an obstacle, it may happen to stumble against it, and be disposed to fall down: such an obstacle is a skandalon.
In like manner, while going along the spiritual way, a man may be disposed to a spiritual downfall by another's word or deed, in so far, to wit, as one man by his injunction, inducement or example, moves another to sin; and this is scandal properly so called.
Now nothing by its very nature disposes a man to spiritual downfall, except that which has some lack of rectitude, since what is perfectly right, secures man against a fall, instead of conducing to his downfall. Scandal is, therefore, fittingly defined as "something less rightly done or said, that occasions another's spiritual downfall."
It is a vice opposed to beneficence, the virtue whereby we do good to those in a more precarious or less fortunate position than us. You can read more about Aquinas's views on the sin of scandal at New Advent. Elsberry is exactly right that we Christians should be vigilant against the scandalous, particularly in areas like this where there is a real danger of turning people off of the truth by associating it with something 'less rightly done or said'. It can be a hard line to walk; being diligent in the pursuit of truth is the best guide to walking it.
