Spark

The Spark, short for "the spark of genius", is the mental attribute which is the sine qua non of a mad scientist, who is therefore referred to as "a spark". There can be competent, even brilliant, engineers and researchers who are nevertheless not sparks and therefore will never make the intuitive leaps necessary to "truly warp the laws of nature" and arrive at such creations as Dr. Jekyll's formula, Dr. Frankenstein's monster, a Strong AI implemented in clockwork, or even the absolutely perfect cup of coffee. (However, some apparently sparky creations such as nuclear submarines equivalent to Captain Nemo's Nautilus and heavier-than-air flying machines actually do exist in our world.)

Sparks also tend to inspire loyalty (or at least obedient fear) in lesser mortals and thus aquire a coterie of minions.

This intuitive ability and associated charisma come at a price, however: sparks are mad scientists who will lose awareness of practical and ethical concerns in their pursuit of technical apotheosis.

Aspects
One of the features of the Spark is that each mad scientist has a unique style. At the grossest level this refers to the spark's preferred medium -- which is often fairly easy to describe: Agatha Heterodyne works mainly with clanks and small-to-medium sized weaponry, for example; while Dr. Dimitri Vapnoople appears to have worked primarily with small furry animals.