The word "lahar" is a general term used to describe a flowing mixture of water and pyroclastic debris; it does not specifically refer to a particular rheology or sediment concentration. Lahars can exist as normal stream flows (sediment concentration of less than 30%), hyper-concentrated stream flows (sediment concentration between 30-60%), or debris flows (sediment concentration exceeding 60%). Indeed, the rheology and subsequent behaviour of a lahar flow may vary in space and time within a single event, owing to changes in sediment supply and water supply. Lahars may be described as 'primary' or 'syn-eruptive', if they occur simultaneously with, or are triggered by, primary volcanic activity. 'Secondary' or 'post-eruptive' lahars occur in the absence of primary volcanic activity, e.g. as a result of rainfall during pauses in activity or during dormancy.