A device for measuring current. Webster's more 'commonly used' definitions are obsolete, in my experience.

An ideal ammeter should have zero resistance. They usually do not, but are close.

Old ammeters work by pushing the current through an electromagnet, which can deviate the needle on the display. This works fine for direct current, but be careful when measuring something that changes over time, as the electromagnet is equivalent to an inductor. And be very careful when you try to use an old-style ammeter to measure Alternating current. Old-style ammeters can do this, but only if you hook them up right. If you don't, you might break the device, as the needle starts vibrating back and forth at 60Hz.