Broadly speaking, there are two general definitions of "deviant."
In common language, it means sick or disgusting, and usually has a sexual overtone. In that circumstance, a deviant act would be a sexual act that the speaker disapproves of. Most contemporary societies consider sex between an adult and a child to be deviant, and their laws reflect this perspective.
Now, from a sociological perspective, "deviant" doesn't carry that same sense of disgust or moral disapproval. A deviant act is just one that is statistically uncommon or breaks a society's rules. When men started to wear their hair long in the 1960s, that was considered a deviant act. It broke American society's rules for what was considered appropriate or acceptable male action. However, in this broader view, deviant acts might also be those that are simply strange to a society, like someone choosing not to have a cell phone.
A deviant act is something that breaks socially accepted rules. They can vary from culture to culture based on existing cultural norms. So something that is deviant in one society, can seem perfectly normal in others. Deviance also varies among generations and age groups, younger generations have a much different view on deviant behavior than middle age or old age groups. An example of a deviant act in the past that is acceptable now is women wearing pants or working while married. Both are seen as non-deviant now that our society has changed. Deviant acts today that are broadly agreed upon in all age groups are violence, especially violence against children. A deviant act in the United States would be stepping on the U.S. flag. Formal deviance is anything that breaks the law and is considered a crime. Deviant acts among teenagers can be something as simple as breaking a social rule or wearing the wrong type clothing.