I’ve heard the term “scandal” used in these forums. What does that word mean in Catholic circles?
There is a particular sin that is giving scandal. This means that a person acts in such a way that others are led to believe that something bad (sinful) is actually not. The sin is usually applied to those in a position of influence and the victims are those who rely on them for guidance and example. Giving scandal often leads another into sin.
By way of example, let’s say a couple are living together before marriage. They aren’t having sex but have set up a common household. One has a younger sibling who, not knowing the intimate details of the couples bedroom, now comes to think that living together as an unmarried couple is just fine.
Even though the couple are not sinning by engaging in premarital sex, they may be committing the sin of giving scandal to the younger sibling.
I have trouble understanding this as well. I was married when I became pregnant with my son, but public opinion was that I still looked about 14. (I frequently had clients stare at me hesitantly and then ask if I was old enough to be working) Was I giving scandal by appearing to be a teen pregnancy? I never understood how much responsibility lands on us for what others might imagine about our lives.
In Catholic theology, the sin of scandal has a particular meaning that does not line up 100% with the way people use the term in popular parlance (including on this forum). Here is how the Catechism of the Catholic Church explains it:
**Respect for the souls of others: scandal **
2284 Scandal is an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor’s tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense.
2285 Scandal takes on a particular gravity by reason of the authority of those who cause it or the weakness of those who are scandalized. It prompted our Lord to utter this curse: “Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea.”  Scandal is grave when given by those who by nature or office are obliged to teach and educate others. Jesus reproaches the scribes and Pharisees on this account: he likens them to wolves in sheep’s clothing. 
2286 Scandal can be provoked by laws or institutions, by fashion or opinion.
Therefore, they are guilty of scandal who establish laws or social structures leading to the decline of morals and the corruption of religious practice, or to “social conditions that, intentionally or not, make Christian conduct and obedience to the Commandments difficult and practically impossible.”  This is also true of business leaders who make rules encouraging fraud, teachers who provoke their children to anger,  or manipulators of public opinion who turn it away from moral values.
2287 Anyone who uses the power at his disposal in such a way that it leads others to do wrong becomes guilty of scandal and responsible for the evil that he has directly or indirectly encouraged. “Temptations to sin are sure to come; but woe to him by whom they come!” 
 Mt 18:6; cf. 1 Cor 8:10-13.
 Cf. Mt 7:15.
 Pius XII, Discourse, June 1, 1941.
 Cf. Eph 6:4; Col 3:21.
 Lk 17:1.
Primarily, it is about leading others to sin through our actions. Many seem to use it simply to describe behavior they find morally questionable. That’s not exactly the same thing.
No you are not responsible for other’s viewpoint unless you are participant in a behaviour that can lead someone to erroneous interpretation.
If you look too young it is not your fault and therefore cannot be counted against you.
We are called to live virtuouse lifes and be of example to others, something that apparently nowdays people consider unattainable, the opposite of which could become scandalous
I think this illustrates well what many think of scandal which is not actually scandal (in the technical sense). If someone looks at a person’s externals and condescendingly thinks this person must be a big ol’ sinner, there is not really “scandal” going on because they aren’t being tempted to follow suit. They are simply committing the sin of rash judgment.
Many times people use the word “scandal” when what they really mean is that an action offends their moral sensibilities.
I agree that the concept of “scandal” is often applied wrongly. Very often, it is used to “make a sin” out of an act, when that act is actually not a sin.
We should avoid crying “scandal” just because we don’t like a particular act. Just because an act HINTS at the existence of sinful conduct, does not mean that scandal is given.