Unfortunately, no definition is required as parties are fully aware if their family law matter is high conflict or not. Conflict is highly detrimental to parties as it causes stress, anxiety, frustration, anger, etc.
Usually, parties operate together with certain dynamics/cycle and it is hard to change such. Separation in of itself does not change the dynamics/cycle.
When there is high conflict, parties negotiating without a go-between is often discouraged. It is counter-intuitive, but having a lawyer as the go-between usually decreases the conflict and can decrease the separation costs as the process moves forward faster to a resolution.