Ammeter
Definition
An ammeter measures electric current — the actual flow of charge. It must become part of the circuit, wired in series.
Technical definition
An ideal ammeter has zero resistance so it doesn't reduce the current it's measuring. To use one, you break the circuit and insert the meter so all the current flows through it.
Example
A phone charger cable carrying 2 A will read 2.00 A on a meter placed in series with it — the same current flows through every point of that single loop.
Common confusion
An ammeter must never be connected across a battery or component (in parallel). Its near-zero resistance would let a huge current surge through, blowing its fuse or the meter itself — that job belongs to the voltmeter.