The number of updates (ticks) the game performs per second. UPS optimization is often referring to getting more performance out of the same level of production.
Refers to the act of having one crafting machine's (furnace, assembler, etc.) output inserted directly into another crafting machine with no intermediate belts or trains.
Having two inserters with a chest(or car/ stationary wagon) in between is also considered DI.
Refers to the situation where an inserter has swung into its drop target and is waiting for available space to drop the remainder of its items.