A millimetre is a unit of measurement used to measure very small distances. It is equal to one thousandth of a meter or approximately the thickness of a dime or a paperclip. Full definition
This walking technique improved in 100 bichir fishes that Standen and her colleagues reared in a tank filled with just a couple of millimetres of water. (newscientist.com)
At 34 days, the resulting organoids were only a few millimetres in diameter and had no blood cells, immune cells, nor the ability to process food or secrete bile. (scientificamerican.com)
Place another piece of grease proof paper on top and, using a rolling pin, roll the pastry put until a few millimetres thick. (hognutsbutters.com)