A tendency can be shown to be present in all known categories of samples (justifying 'Iron') , while not occuring in every single sample, making it a tendency.
Separately, tendency can mean a type of outcome is more likely, or that there is an underlying force acting in a direction, regardless of whether that force is cancelled by other forces.
Separately, tendency can mean a type of outcome is more likely, or that there is an underlying force acting in a direction, regardless of whether that force is cancelled by other forces.