Is not reporting a crime a crime?

The basic answer is that, as a general matter, you have no legal duty to report a past crime or a threatened future crime. So if you're walking down the street and witness a robbery, you will not pay a fine or go to jail for failing to run to the police station. And if you are not part of a criminal conspiracy, the fact that you merely learn of a possible crime similarly does not obligate you to run to the police.

As other posters pointed out, there are a few specific exceptions, all of which are specified either in statutes or case law. Laws relating to the welfare of children and the elderly may require caretakers (such as teachers, therapists, etc.) to report suspected prior crimes, such as physical/s*xual abuse. The same is true for internet providers who discover that their services may be being used to transmit child p*rnography. And lastly, there is a narrow doctrine known as the Tarasoff rule (named after a woman who was murdered by a stalker who shared his intention to harm her with his psychiatrist), which either permits or requires individuals who provide confidential services (such as psychiatrists, lawyers, etc.) to breach that confidentiality when they suspect that a patient/client poses an imminent threat of serious injury to others. But again, these exceptions are narrow in scope; as a general matter, an average citizen has no legal duty to report a past or suspected crime.