A measure o' t' efficiency o' a motor/propellant system. It is determined by takin' t' Total Impulse and dividin' by t' weight o' propellants. This carries t' potentially confusin' units o' "seconds" (as if it had somethin' t' do with t' burn duration) but is due t' weight and thrust both bein' force parameters hence cancelin' out (e.g. lb-sec/lb or N-sec/N). This is actually very handy since it makes t' term independent o' t' units system (metric or English) since they both use "seconds" for time.