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I find it helpful (like Herb evidently does) to think in terms
of science keywords.
So density starts out as a particular ratio:
e.g the ratio of mass per unit volume
The idea is generalized to
energy density - the energy per unit volume
charge density - the charge per unit volume
and (incorrectly) to
current density - the current per unit volume
when really, the current per unit cross-sectional area, is meant.
But this usage is by now enshrined and won't be shamed
from existance.
specific impulse: impulse per unit fuel mass,
derived in a slightly tortured way from force or thrust per
unit fuel mass flow rate.
This too enshrined in custom as a 'time' by dividing units of
force by units of mass - not always wise.