Within Conway's Life, the period is number of generations that it takes for something to re-appear in its original form. In its true form, this applies only to oscillators and spaceships. The most common oscillator is that of the blinker which has a period of 2:
... .o. ...
ooo -> .o. -> ooo
... .o. ...
still life is considered to have a period of 1.
The term of period can be applied to other arrangement of cells within Conway's life that don't exactly fit the definition of a true period - a puffer, wick, fuse, superstring, stream of spaceships, factories, and guns all exhibit something that resembles a period. However, in these cases, the arrangement of cells after one period isn't exactly the same - something has changed. With puffers, guns, and factories, something has been produced (see glider gun for an example - it is period 30), while wicks, fuses, and super string contain fewer cells. Thus, these periods are considered pseudo-period rather than a true period.