Q: “What’s the difference between gain and trim? I see these words used almost interchangeably.”
They are used interchangeably quite often, though there are subtle differences to what they connote. As you will find when you look the terms up in our WFTD archives (you do check out these definitions, right?) gain refers to how much a signal is amplified as it passes through a circuit or device. Trim refers to changing the level of a signal (up or down) to more appropriately align it for a particular device. A trim control may apply some “gain” to raise a signal level, or it may apply some negative gain, or attenuation to lower a signal. The initial gain setting on many mixers may be called gain or trim depending on the manufacturer’s conventions. Both terms can be generally correct in that context.
Trim also somewhat implies a fine adjustment. This may apply to getting two line level devices to match levels perfectly (requiring small adjustments) or getting two subassemblies in a circuit to match, or any of dozens of other similar scenarios. Gain, while often acting as a trim control, doesn’t necessarily connote small or fine adjustments (though they are often possible as well). In fact, some mixers have both a coarse and fine gain control. In that context the fine control might be called trim, but this again depends on the manufacturer.
In general any time you are referring to relatively fine adjustments the word trim can apply. Anytime you are referring to boosting a signal the word gain can apply. These aren’t the only times these words apply, but in general this is how they are most commonly used. As you can see there is substantial overlap between the two.