Mikoportal is the name given to a set up in which a floor, at the lowest height possible in the vanilla Doom engine, is connected to its ceiling, which is itself at the highest height possible, through arithmetic underflow. If the player's avatar (or a voodoo doll) falls towards such a floor, its vertical position will underflow to reach the top, allowing to generate an endless fall. While a player character does not touch the floor, the engine does not apply friction, therefore momentum is preserved until reaching a wall or landing on a floor at a higher than minimum height.
It was discovered in November 2018 by Mikolah who was the first person to have the idea of exploiting this phenomenon to achieve special effects not normally possible in the engine, notably vanilla conveyors. The Mikoportal is named after him. It originated in a Doomworld Forums thread titled "Did you know if you fall onto a floor at -32768 units it turns into a portal" by Andrew Stine (Linguica).
Mikoportals are made possible by the implementation of actor coordinates in Doom being expressed as 16.16 fixed point numbers, represented internally as 32-bit integers. The effect is therefore absolutely incompatible with any port that uses floating point coordinates, or moved to 64-bit integers to represent fixed point. If needed, the effect can be emulated or replaced easily with advanced modding features.