An aqueduct is any combination of elements that carries water from one location to another. It can use channels, tunnels, and/or natural or constructed floors and walls to create the path for the water to follow, and can tap into the side of a natural water source or use pumps and gravity to get the water to flow to its desired source. It may need to be roofed over to create a sealed system if pressure is involved (Dwarf Fortress water pressure differs from real-life physics, and should be understood before undertaking large fluid-moving projects to avoid flooding your fortress).
Similar techniques can be adapted to move magma across the map.
In the old 2D versions (v0.23.130.23a and earlier) of Dwarf Fortress, aqueducts were a specific construction, much like a bridge, and such references may be found in older discussions.