Just for the record it has a somewhat broader meaning in tube amps. It does not refer to the connection between the output stage and the speakers. (Conventional tube amps all use transformers)
With tube amps it refers to how the stages (separate gain circuits) of a tube amp are connected (coupled). Usually a resistor and capacitor are used. You can use an inductor or a transformer too. The cap or transformer blocks the dc plate voltage of stage 1 from getting to the next stage (at the grid). In a directly coupled amp the plate of stage one is connected to the grid of stage 2 with no cap or transformer in the path, so dc is allowed to pass. There is nothing between the plate (of stage one) and grid (of stage 2) so the connection is "direct."
This makes it a bit more complicated as the grid of stage 2 has a lot of dc on it. The 2nd stage requires considerable higher voltage. On the good side you do not get the phase distortion, time delay or frequency loss that is introduced by a cap. Some think it sounds best.
With tube amps it refers to how the stages (separate gain circuits) of a tube amp are connected (coupled). Usually a resistor and capacitor are used. You can use an inductor or a transformer too. The cap or transformer blocks the dc plate voltage of stage 1 from getting to the next stage (at the grid). In a directly coupled amp the plate of stage one is connected to the grid of stage 2 with no cap or transformer in the path, so dc is allowed to pass. There is nothing between the plate (of stage one) and grid (of stage 2) so the connection is "direct."
This makes it a bit more complicated as the grid of stage 2 has a lot of dc on it. The 2nd stage requires considerable higher voltage. On the good side you do not get the phase distortion, time delay or frequency loss that is introduced by a cap. Some think it sounds best.