Some key items in metrology are listed below. Please click to find their definitions according to the Internationale Vocabulaire VIM. 

• (measurable) quantity
• unit (of measurement)
• value (of a quantity)
• measurement
• uncertainty of measurement
• error of measurement
• measuring instrument
• material measure

(measurable) quantity
Attribute of phenomenon, body or substance that may be distinguished qualitatively and determined quantitatively.

Examples
a) quantities in a general sense: length, time, mass, temperature, electrical resistance, amount-of-substance concentration
b) particular quantities like a length of a given rod or the electrical resistance of a given specimen of wire

unit (of measurement)
Particular quantity, defined and adopted by convention, with which other quantities of the same kind are compared in order to express their magnitudes relative to that quantity.

A system of units (of measurement) is:
a set of base units, together with derived units, defined in accordance with given rules, for a given system of quantities, such as the **International System of Units, SI

value (of a quantity)
Magnitude of a particular quantity generally expressed as a unit of measurement multiplied by a number.

Examples
a) length of a rod: 5,34 m or 534 cm
b) mass of a body: 0,152 kg or 152 g

measurement
Set of operations having the object of determining a value of a quantity.

uncertainty of measurement
Parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand.

The parameter may be, for example, a standard deviation (or a given multiple of it), or the half-width of an interval having a stated level of confidence.

error (of measurement)
Result of a measurement minus a true value of the measured.

measuring instrument
Device intended to be used to make measurements, alone or in conjunction with supplementary device(s).

material measure
Device intended to reproduce or supply, in a permanent manner during its use, one or more known values of a given quantity.

Examples
a) a weight
b) a measure of volume (of one or several values, with or without a scale)
c) a standard electrical resistor
d) a gauge block
e) a standard signal generator
f) a reference material