Regulators in different countries will define accidents and incidents differently. However, in a broad sense we can think of incidents and accidents being very serious, or less serious.
In general, if the incident/accident has resulted in damage to property or the death or severe injury of a person, this will be considered a very serious event. If there has been no injury to a person or damage to property this may be considered less serious.
However, there are often exceptions. For instance, if there is a near miss with a manned aircraft, this is likely to be considered a very serious incident.
You should also consider that it is not just aviation regulators who may have oversight of your flight operations. If your operations are taking place over a mine, or a construction site, for example, there are likely to be other regulators who have jurisdiction and may have different definitions of what is a serious incident and what is a less serious incident.
Requirements for accident and incident reporting
All aviation regulators have requirement in place about when you need to report an incident or accident to them. It is important to make sure you know what these are and have systems in place to ensure you are compliant with them. As above, there may also be other regulators who you need to report incidents and accidents to, depending on the industry in which you operate.