A felony is a serious crime.
The X-Men committed a felony when they attacked the Mutant Control Agency. Beast was captured and brought to trial for the crimes. His defense lawyer Hodge argued that he was justified in committing the felony.
Kingpin committed a felony when he sold government secrets. Kingpin's son, Richard Fisk, later framed Peter Parker for his fathers crimes. However, Peter was later cleared of all charges and Richard Fisk was arrested.