There are two common terms when describing sensors. One is “degree of freedom” (DOF) and the other is “axis.” They're often used synonymously, originating when accelerometers, vibration sensors and tilt sensors were first used in industrial and military applications for monitoring movement, such as a robotic arm or a space craft. Yet, they don't mean the same thing. Technically, a DoF is a parameter that determines the state of a physical system and it takes six numbers to characterize a movement.
So, six degrees of freedom indicates the sensors track up and down, side to side, forward and backward, pitch, roll, and yaw. A device can really only have a maximum of 6 DOF, because there are only 6 degrees of freedom in a 3D space. When a company claims a higher DOF (like 9), they’re really implying a higher degree of accuracy within the linear and angular 3D space.
Which means the more accurate term when discussing sensors is "axis
The axis term refers to the X, Y, and Z axis, so a 1-axis sensor most likely goes up and down on a Y axis or side to side on a X axis. A 3-axis sensor would track on all three axes. A sensor platform that is said to have 9, 10, or 12-axes indicates that it tracks multiple data points along the X, Y, and Z axes. The number of axes a device is said to have can be as high as the developer wants - as long as each additional axis tracks data along one of the x, y, or z axes, it can be added on to the total.