I don't understandNo. It cannot be both perpendicular and in the same direction. That is a contradiction.
In this case, criticism is not merited. The use of the indefinite article is plainly wrong at first glance, but in fact there is an infinite number of horizontal axes perpendicular to the direction of travel.Not to criticize the Oxford dictionary's definers too much, I personally think that the definition would have been clearer if it said "...around the horizontal axis perpendicular to the direction of motion", since this axis is uniquely determined.
Pitch: bow goes up and down.The three forms of motion around these axes are called pitch, yaw, and roll, though I can never remember which is which
Roll - movement around the direction of motion.The direction of motion is one horizontal axis, and there is another one perpendicular to it, making a cross in the horizontal plane. The third axis is vertical. All three axes are perpendicular to each other. The three forms of motion around these axes are called pitch, yaw, and roll, though I can never remember which is which (and have to make embarrassing hand gestures to try and visualize it).