Prefix
In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10 (short scale trillionth or long scale billionth). Symbol: p
(biology) Used to designate sizes, especially, of cells, smaller than those prefixed by nano- or micro-; sized between 0.2 and 2.0 μm.
Source: en.wiktionary.org