Resonator

It could reasonably be argued that all Ukuleles, for that matter all Chordophones are "Resonators" as all of them rely on resonance from the strings to create a sound, but for the purposes of this Wiki there are two distinct and specific meanings for the term "Resonator"; Banjo Resonators and "Resonator Instruments"

Banjo Resonator
When talking about Banjo type instruments the term "Resonator" is used to describe the, usually removable, wood or mental, flat or lipped, plate or dish connected to the back of the drum with a small gap between it and the drum.

Sometimes the Resonator pan is bigger than the Banjo drum and the gap here is often covered by some sort of metal grill. Sometimes it is the same size and spacers are used to create the gap.

Resonator Chordophones
The reason for the creation of these "Resonator" was, before the days of electric amplification, the volume of the various chordophones was not great enough to compete with the aerophones of the time and was not really capable of being heard clearly in larger venues. Orchestras compensated for the failings with numbers which is why the string section of an orchestra is the largest. The first technology based method to try to combat this and improve volume was the Banjo and the various Banjo type instruments. However the Banjo was more of a solo instrument and the tonal differences between the various instruments and their Banjo enhanced derivatives was such that other, more natural sounding alternatives were sought.

Gélas aka Double-Top, Double Resonance
This is a design using two soundboards with a resonating space between them. The inner soundboard is not parallel to the outer one and the strings run through the bridge, (which is on the inner sound board), and are attached at the bottom, so the plucked part of the string before the bridge is at an angle to the part of the string after the bridge.

This design was patented in France by Lucien Gélas in 1905 for Guitars but it was used by him, and other luthiers on Ukuleles and Mandolins as well