In the States we often call them "button" connectors because they are like small buttons. Incredibly popular with the landline telcos because they are faster and more reliable than the crimp sleeve connectors that were previously used, mainly on 24g wire.
The "normal" premium grade ones are also always filled with silicone jelly, so the wire connection is automatically sealed against oxidation and corrosion
. And there's no need to strip insulation
from the wires, because this is a "vampire" clamp that cuts through the insulation to contact the bare wire. That's a bad thing if you need to carry large amounts of current
(because it is only "edge" contact) but a good thing as long as you're using the correct wire size and it is a low power