Accelerated Test 
A test in which the applied stress level is chosen to exceed that stated in the reference conditions in order to shorten the time required to observe the
stress response of the item or magnify the response in a given time. To be valid, an accelerated test must not alter the basic modes and/or mechanisms of failure
or their relative prevalence. 
Acceleration Factor 
The ratio between the times necessary to obtain a stated proportion of failures for two different sets of stress conditions involving the same failure
modes and mechanisms. 
Accelerometer 
A device ( a sensor or transducer or pickup) for converting x” to an electrical signal. 
Availability 
The ability of an item (under combined aspects of its reliability, maintainability and maintenance support) to perform its required function at a stated instant of time or over a stated period of time.

Balancing(mechanical)

Adjusting mass distribution in a rotating element, to reduce vibratory forces generated by rotation. 
Constant Failure Period

That period during which failures of some items occur at an approximately uniform rate. 
Critical frequency 
A particular resonant frequency at which damage to (or degradation in performance) is likely., see resonance. 
Damping

Dissipation of oscillatory or vibratory energy with motion or with time. Critical damping is that value that provides most rapid response to a step
function without overshoot. Cc=2 (KM)^{1/2}_.Damping ratio is then C/Cc 
Decade 
the interval between two frequencies which differ by exactly 10:1 
Decibel 
Ratios of identical quantities are expressed in decibel or dB units. Magnitude thus refers to some standard value, in terms of the base 10 logarithm of
that ratio. 
Degrees of freedom 
In mechanic, the total number of direction of motion (of all the points being considered) on a structure being modeled or otherwise evaluated. In
statistics, the number of independent variables used in constructing a mathematical model representing some collection of random variables. 
Deterministic Vibration 
A vibration whose instantaneous value at any future time can be predicted by an exact mathematical expression. Sinusoidal vibration is the classic
example. 
Displacement 
It specifies change of position, usually measured from mean position or position of rest. Usually applies to uniaxial, less often to angular motion. 
Distortion 
In mechanics, any unwanted motion. If sinusoidal motion were desired at a fundamental frequency, any ,motion at harmonics or sub harmonics of that
frequency, or any mechanical ‘hash’ (perhaps due to parts colliding). In electronic measurement, any unwanted signal; e.g. amplifiers may generate unwanted
signals. 