The<unk>  of an object about a given axis describes how difficult it is to change its angular motion about that axis.  For example, consider two discs (A and B) of the same mass.  Disc A has a larger radius than disc B.  Assuming that there is uniform thickness and mass distribution, it requires more effort to accelerate disc A (change its angular velocity) because its mass is distributed further from its axis of rotation: mass that is further out from that axis must, for a given angular velocity, move more quickly than mass closer in.  In this case, disc A has a larger<unk>  than disc B.
The<unk>  of an object can change if its shape changes.  A figure skater who begins a spin with arms outstretched provides a striking example.  By pulling in her arms, she reduces her<unk> , causing her to spin faster (by the conservation of angular<unk> um).
The<unk>  has two forms, a scalar form 
I
{\displaystyle I}
 (used when the axis of rotation is known) and a more general tensor form that does not require knowing the axis of rotation.  The scalar<unk>  
I
{\displaystyle I}
 (often called simply the<unk> ") allows a succinct analysis of many simple problems in rotational dynamics, such as objects rolling down inclines and the behavior of pulleys.  For instance, while a block of any shape will slide frictionlessly down a decline at the same rate, rolling objects may descend at different rates, depending on their<unk> .  A hoop will descend more slowly than a solid disk of equal diameter because more of its mass is located far from the axis of rotation, and thus needs to move faster if the hoop rolls at the same angular velocity.  However, for (more complicated) problems in which the axis of rotation can change, the scalar treatment is inadequate, and the tensor treatment must be used (although shortcuts are possible in special situations).  Examples requiring such a treatment include gyroscopes, tops, and even satellites, all objects whose alignment can change.
The<unk>  can also be called the mass<unk>  (especially by mechanical engineers) to avoid confusion with the second<unk>  of area, which is sometimes called the<unk>  (especially by structural engineers) and denoted by the same symbol 
I
{\displaystyle I}
.  The easiest way to differentiate these quantities is through their units.  In addition, the<unk>  should not be confused with the polar<unk> , which is a measure of an object's ability to resist torsion (twisting).
Moment of inertia
Pressure gradient 
Density
Volumetric flow rate
Molecular mass
Electrochemical gradient
Extraction ratio
Fluency