Arc Second 
The prefix, arc,
is used solely to differentiate this angular measurement from a
second of time. By convention, it is 1 part in 1,296,000 of a
circle; or 1 part in 3,600 of a degree; or 0.0002778 degree.

Error Bandwidth 
The total
measure of deviation between all readings of an instrument
throughout its measuring range.

Error 
The difference
between a measured value and the true value.

Flatness &
Wobble 
A composite
tolerance applied to a flat surface rotating about an axis, e.
g., the faceplate of a rotary table. It is defined such that the
entire rotating surface lies between two parallel planes
separated by a distance equal to the tolerance specified.

Hysteresis 
A bias resulting
from the reversal of direction.

Index Accuracy 
The agreement of
the result of an angle measurement between the actual reading of
the instrument and the true value, that is free of other error,
at that point.

Latent Motion 
The measure of
change, as measured at the platen in the axial and radial
directions, between the unclamped and clamped condition on a
rotary table equipped with axis clamps.

Parallelism 
The state of two
planes parallel to each other; where two planes are
equidistantly spaced in three dimensional space.

Perpendicularity 
Condition of a
line in which all angles to a reference plane are at right
angles.

Repeatability 
The measure of
accuracy by which an instrument permits the return to a specific
point. In a normal or Gaussian distribution, the results are
spread roughly symmetrically about the central value, and small
deviations from this central value are more frequently found
than the large deviations. The normal curve can be represented
by
The standard
deviation, denoted by
is found by taking the difference between each observed
particular value and the mean, then squaring the difference,
adding all the squares, dividing by the number of readings, and
then taking the square root.

Resolution 
The smallest
increment of measure to which rotary table can respond.

Rotary Axis
Definition 
A measurement
which compares two full consecutive axis rotations to a known
standard of roundness.

Roundness 
A characteristic
that all parts of a circle are identical. The measurement of
roundness is essentially a measurement of the change in radii,
and the roundness error is the measurement between the minimum
and maximum radii in one lateral plane.

Runout T.I.R. 
Total indicator
deflection as measured over one revolution of the rotary table.

Wobble (Axis) 
The measure of
deviation of a rotating plane to a reference plane. The
measurement is usually performed with a mirror and an
autocollimator. 