On a bicycle, a internal gearbox is a gearbox that's located inside the bicycle frame, above the down-tube. This is very similar to how a motorcycle transmission is mounted.
Many framebuilders have experimented with this design over time, but haven't rolled out bikes to production, because they tend to fairly heavy.
The Zerode G-1 is a bicycle that incorporates a internal-gear-hub that's been modified to sit inside the frame.
The Pinion P1.18 is actually located near the bottom bracket. However, it requires a substantially modified frame — the bottom bracket area of the frame is very different.