Attitude is rotation of the body frame of reference (its principal axes, whatever it means) relative to a chosen frame of reference.
The body frame of reference is the longitudinal, lateral and vertical axes, where the longitudinal-vertical plane is the plane of symmetry and longitudinal axes is chosen to point generally forward, along cabin floor if there is such a thing.
The chosen outside frame of reference is the local horizontal, which is plane perpendicular to the local weight¹ vector, and in that plane the main axis is chosen either as projection of either local magnetic inductance vector (magnetic heading) or Earth rotational axis (true heading).
Regarding “true horizon”, in the sense of where the you see the ground/sky boundary, that can't be used for defining attitude at all, because it is not a plane. The line connecting you to the “true horizon” ahead and the line connecting you to the “true horizon” behind is not the same line, so it can't be used as axis.
Instead, horizon is defined in terms of horizontal, the plane orthogonal to the weight vector (and tangent to the Geoid, the ‘idealized’ sea level).
To fly level, your velocity vector must point along the local horizontal. In most aircraft it means your pitch is slightly positive. This is because most aircraft are designed to never need to fly at negative pitch² and as the angle of attack for level flight depends on weight and speed, most of the time your pitch will be slightly positive.
¹ Not “gravity”, because it centrifugal force due to Earth rotation is included. Actually, the instruments measure the value including the centrifugal force due to the movement of the aircraft, which might be slightly different from centrifugal force on stationary object. The immediate measurement also includes the inertial force due to acceleration (the forces can't be distinguished), but this is eliminated by averaging, because the average acceleration (over several minutes) is known to be zero or close enough for practical purposes.
² There are some exceptions that do normally fly slightly nose-down, like B-52, which does it because its unusual gear does not allow it to rotate on take-off, or DHC-6 Twin Otter, which apparently does it to provide better view during slow short take-offs and landings.