Alex Trebek, "Little black boxes maybe 4-5 in square and an inch thick."
Me, "What is, apple tv?!"
Damn.
For reals though, it's exactly what your title says, "little black box that replaces my tower" --- it IS the tower, just smaller.
I'd only refer to this as a node if the computer itself isn't doing the processing. Then it would basically be just a medium for i/o (input/output... keyboard, mouse, monitor, usb).
Your company may be doing it like that but more likely your computer is still running the software physically.
A notable example of this type of computer is the Intel Nuc
They can be spec'd as low as $250 (with a Celeron J4005 - 2 threads) and go all the way up to north of $1600 with (a Xeon E-2286M - 16 threads!) -- you can also get kits that have no memory and drive for like $135. -- can make for some pretty dense computing.
There are all kinds of mini PCs like this. Search Amazon for "mini PC" and watch some youtube reviews to find a good one at a good price. It's ever changing.
In 2015 I may have said that this could replace your tower with something like this for lighter workloads like general browsing. Today... you can get a lot done in a much smaller package. Having said that, if you spec'd a full tower at the same price you'd have WAY more power. ...can't really fit an RTX in one of these. -- also I feel it's a huge waste of a processor if you aren't going to take advantage it's extra PCIe lanes. But there's something to be said for such a nice form factor. There are many use cases where this is a much better solution.