Acceleration = the rate of change of velocity per unit of time
That's Physics for you.
Sorry, thats bollox for you.
acceleration is by definition the rate of change of velocity, as measured in an inertial frame of reference (ie a non-accelerating frame). The issue people seem to be having is what is 'velocity', which I think is the root of the differences of opinion.
The conventional frame for measuring Velocity is the ground (which is very close to inertial after adjusting for a 1 g offset from gravity). So, if an aircraft has changed its ground velocity, it has by definition accelerated.
If the air mass is moving at a constant velocity (with respect to the ground), measuring velocity relative to the air mass will give exactly the same acceleration as measuring vs the ground.
If you do the maths for two cases of a level constant rate turn
1 - flying North at 60 knots and turning to South at 60 knots
2 - flying North at 0 knots (into a 60 knot headwind) and turning to South at 180 knots
You will find the answers are all identical, except the track over the ground is displaced by a constant distance of 60 nautical miles every hour in the second case.
In both cases the air SPEED (not air VELOCITY) will remain constant. The Relative Air Velocity (measured relative to the wing, which for normal rates of turn is effectively an inertial frame) will remain constant. It is this relative Velocity that defines the performance of an aircraft wing.
If the air mass is not moving at a constant speed, the aircraft will require accelerations to maintain the relative air velocity, hence the importance of the aircraft's inertia when exposed to gusts and shears.