What is the meniscus

meniscus |məˈniskəs|

noun (pl. menisci |-kē, -kī| or meniscuses)

Physics  the curved upper surface of a liquid in a tube.

A cup of water, measures 8 fluid ounces, but does not weigh 8 ounces. Look up water in the dictionary. It defines one fluid 8 ounce cup of water as 236.6 grams (8.3 ounces weighed). The volume reading should be taken at eye level and the meniscus--the clear space at the very top--should be above the measuring cup’s marked line.

If one mistakenly measures with the top of the line level with the cup’s, one can be short as much as one tablespoon. Shown below at 223 grams.

(Incidentally, liquid measures are not designed to measure solids such as sugar and flour which need measuring cups with unbroken rims on which to level off the ingredient.)