What is a Dalton?

We put this question to Dr Harren Jhoti:

Harren - Basically a Dalton is just a measure of weight, it's a measure of mass. It's a very, very small measure of mass: something like micrograms or grams people refer to but at a kind of microscopic level, if you like. Daltons are basically very single units of small mass.

Chris - So when you're down at the molecular level you can't use grams or even thousandths of grams? You need to use much smaller molecular units?

Harren - That's exactly right. That's the most commonly-known unit which people refer to when looking at small molecules.