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In TOS, "impulse" does not always refer strictly to sublight engines; this is noted in "Where No Man Has Gone Before" (the previous ship to get lost at the edge of the galaxy was an impulse vessel) and "Balance of Terror," in which the Romulan ship is described as having "impulse" power rather than warp power.

Presumably, this means the vessel had fusion, rather than antimatter, power systems.

How much acceleration impulse provides is unclear in TOS.