mass

Mass is the quantity of matter present in an object or substance, usually measured in grams or kilograms.

• Mass is often confused with weight, but they're different: weight is the force with which Earth pulls an object toward itself (measured in newtons), while mass stays the same wherever the object is.
• Most balances actually measure weight, but their scales are marked in grams or kilograms anyway, since weight and mass are so closely tied together on Earth's surface.
• A two-pan balance is the one common exception that genuinely measures mass directly, by comparing an object against known reference masses rather than sensing the pull of gravity.
• Mass is one half of the density formula — pairing an object's mass with its volume is exactly what turns "how much stuff" into "how tightly packed that stuff is."