It's an inline barrel connector. There are lots out there, with a plethora of different sizes for the various diameters for both the hole and the pin (if any) which is in the hole. I doubt we can help much more than that.
There's a wikipedia entry for them, they are that common.
If you need to get something which mates with this one, and you can't find a datasheet or formal description, you options are either to try and measure it accurately, or trial and error with a bunch of similar looking connectors.
Don't forget, the internal pin length and girth vary along with the diameter and length of the main orifice.