A group, set, or series of ten , particularly:
a decade of soldiers
A set of resistors, capacitors, etc. connected so as to provide even increments between one and ten times a base electrical resistance.
The interval between any two quantities having a ratio of 10 to 1.
There are decades between 1.8 and 18, between 25 and 250 and between 0.03 and 0.003.