Parts per notation - ppm, ppb, ppt
"Parts per" is a convenient notation used for low and very low concentrations. Generally speaking it is very similar to weight by weight percentage - 1% w/w means 1 gram of substance per every 100 g of sample and it is (although very rarely) named pph - parts per hundred. Other abbreviations stand for:
|ppm||parts per milion (106)|
|ppb||parts per bilion (109)|
|ppt||parts per trillion (1012)|
|ppq||parts per quadrilion (1015)|
ppq is rather a theoretical construct than a useful thing, chances are you will never see it in use.
ppt can be confusing as it is sometimes used for parts per thousand - if you want to use "part per" notation in this case it is safer to use ppth abbreviation (but I doubt you will see it often) or "pro mille" ‰ sign.
Note that "Parts per" notation can be sometimes misleading. ppm usually means 1 mass part per 106 mass parts, but it can be volume by volume, or even 1 particle per 106 particles. 1 mL of SF6(g) added to 1000L of hydrogen H2(g) is 1 ppm vol/vol but 73 ppm mass/mass. In the beryllium sample containing 1 atom of lead per 106 beryllium atoms there are 23 ppm of lead mass/mass.