Page "Specific impulse" Paragraph 3

Propellant is normally measured either in units of mass or weight.

If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to be a unit of speed, and so specific impulses are often measured in meters per second and are often termed effective exhaust velocity.

However, if propellant weight is used instead, an impulse divided by a force ( weight ) turns out to be a unit of time, and so specific impulses are measured in seconds.

These two formulations are both widely used and differ from each other by a factor of g, the dimensioned constant of gravitational acceleration at the surface of the Earth.