Inside the electric bicycle world, monocoque frames are oftentimes referred to frames which houses part or all the electric components.
The word monocoque comes from the Greek for single (mono) and French for shell (coque). In our case, the French term is what we are talking about.
Usually, just the battery is mounted inside the frame but there are a few exception when even the motor is located inside.
There are more degrees of freedom when designing such e-bikes. The designer doesn't have to make a special room for the battery (or the other electric parts if it's full monocoque frame).
He can design the bike to look like a regular non-electric bike with all the benefit that come with it (Aerodynamics improvements, stylish design, less prone to theft and more). At first glance you won't even know that you are looking at an electric bicycles.
For example, a company called GRUBER Assist has such a kit for regular bicycles. There was actually quite a big scandal in the bicycle racing world surrounding the possibility that one of the teams at a specific race used this kind of kits. The news articles called this kit - "the new chemical drug". Here is a link to one of this articles: New Talk Of Cheating.
Famous full monocoque e-bikes are manufactured by Optibike.