What is insidious define?

Insidious is a term used to describe something that proceeds in a gradual, subtle, and often hidden way, especially with harmful effects. It can be used to describe a disease, a process, an idea, or a person's behavior. Insidious behaviors and actions may often go unnoticed or be difficult to detect, making them more dangerous. The term often implies deceit, betrayal, or malice in intent, although it can also be used to indicate circumstances that are unintentionally harmful or negative. Some examples of insidious behavior include gaslighting, manipulation, and emotional abuse, which can have profound and long-lasting impacts on their targets. In popular culture, the term is often associated with the horror movie series "Insidious."