**Early Failure Period** |
That early period beginning at some stated time and during which the failure rate of some items is decreasing rapidly. |

**Failure** |
The termination of the ability of an item to perform its required functions. |

**Failure Mechanism** |
Physical, chemical or other process resulting in a failure. |

**Failure Mode** |
The effect by which a failure is observed: for example, an open or short circuit condition or gain change. |

**Failure Rate** |
At any point in the life of an item, the incremental change in the number of failures per associated incremental change in time. |

**Failure rate, observed** |
For a stated period in the life of an item, the ratio of the total number of failures in a sample to the cumulative observed time on that sample. The
observed failure rate is to be associated with particular and stated time intervals (or summation of intervals) in the life of the item, and under stated
conditions. |

**Forced vibration** |
The vibratory motion of a system caused by some mechanical excitation. If excitation is periodic and continuous, motion eventually becomes steady-state. |

**Free vibration** |
Free vibration occurs without forcing. |

**Frequency** |
The reciprocal of period in seconds (of a periodic function),1/T. measured in Hz. |

**Frequency response** |
The portion of the frequency spectrum which a device can cover within specified limits of amplitude error |

**Frequency spectrum** |
A description of the resolution into frequency components, giving the amplitude (sometimes also phase) of each component. |

**Fundamental frequency** |
The number of cycles per second of the lowest frequency component of a complex, cyclic motion. |

**g** |
The acceleration produced by Earth’s gravity. By international agreement, the value for 1 gravitational unit is 9.80665m/s^{2} |

**g units or gravitational units** |
A way to express x” in terms of the gravitational constant, is equal to x”in/Sec^{2}/386.087 in/ Sec^{2}or to (x”m/sec^{2})/(9.80665m/Sec^{2}). |

**Harmonics** |
A sinusoidal quantity having a frequency that is an integral multiple (X2,X3 etc.) of a fundamental (X 1) frequency. |

**Hash** |
Distortion on a signal viewed on an oscilloscope trace (slang) |

**Impact** |
A collision between masses |

**Impulse** |
The integral of force over a time interval |

**Induced environments** |
Conditions generated by operating an equipment, as opposed to natural environments |

**Instantaneous Failure Rate (hazard)** |
At a particular time, the rate of change of the number of items that have failed divided by the number of items surviving. |

**Isolation** |
A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates
steady-state vibration. |