Right. Along with the clarification added by Procrastinus above, this is one thing I left out for the purposes of simplification. Technically, the TMT model's denominator contains a constant, a measure of delay, and a measure of "susceptibility to delay." 'Impulsiveness' is a decent single-word corallary of 'susceptibility to delay', but may vary depending on the task, whereas 'impulsiveness' is a word that sounds like a stable character trait.