The rigid definition of carnivory in plants requires digestion of prey by enzymes produced by the plant.

With this more rigid definition of similitude, the cylinder and the plane are obviously no longer the same.

Climatologists and geographers have less rigid definitions which take into account factors such as rainfall, temperature, wind direction and ozone.

While the engine and suspension layouts described here for scooters and underbones are typical, they are not rigid definitions.

However, some classes can compete and sail with less rigid definitions and measurements.