TL:DR – Nano means a billionth, a billionth of whatever unit you’re talking about, smaller than that and you are talking pico (a million-millionth), femto (a million-billionth), atto (a billion-billionth), zepto (a billion-trillionth), yocto (a trillion-trillionth). Recently added a thousand times smaller than yocto is ronto and a million times smaller than yocto is quecto.
Use the prefix “nano” and you make any unit a billion times smaller. A nanometre? That’s a billionth of a metre. A nanosecond? That’s a billionth of a second. Nanowatt? Billionth of a Watt. The list goes on, confounded only by that annoyance that is the kilogram, the SI unit of mass. A kilogram being a thousand grams, but a nanogram isn’t a billionth of a gram, it’s just a millionth, it’s a billionth of a kilogram.
Anyway, smaller than nano? Of course…here’s the full list of prefixes that define the thousandths and millionths and billionths of nano…pico (million-millionth), femto (million-billionth), atto (billion-billionth), zepto (billion-trillionth), yocto (trillion-trillionth).
Of course, you can go smaller, although you might not be able to measure the thing you’re measuring and there is quantum mechanics and the Planck limit to take into consideration.
Bigger than nano? Well, we have – kilo, mega, giga, tera, peta, exa, zetta, yotta. Find out more in this Sciencebase blog post from September 1997.
In science and pretty much everywhere now, a billion is defined as 1,000,000,000. In British-English the word billion used to mean a million-million and a thousand-million was archaically known as a milliard.