A kind of ratchet mechanism that works with ropes - a sprung lever that presses against a rope so as to allow it to be pulled through the device in one direction, but digging in and locking when pulled the other way.
I realise there are other, probably more common devices used to lock ropes (I’ve seen devices with little knurled rollers in tracks that cause them to bite down when pulled the wrong way) - but I am trying to find out if this specific one has a name.
A Jumar ascender?
Since the Jumar ascender is a more complex device that includes a pawl, I would go with pawl.
Cool, thanks.
I suppose it might be considered a cam, depending on its shape - the ones in those ascenders look like they would be.
Come-along?
That’s what came to my mind as well.
I vote for pawl as well, as being closer to what’s described in the OP, with its “sprung lever.”
As for the cam cleat illustration Kiwi posted-- I wonder why it was important, when they named those different kinds of cleats, that the names all rhyme? 
Sailors are known for their whimsy.
