A reentrant tuning is a tuning where the strings are not ordered from the lowest pitch to the highest pitch.
A break in an otherwise ascending order of string pitches is known as a reentry. In the case of ukulele, the reentry is between the first and second string. The high g falls between the E and the A (gCEA). An example of linear tuning, when the notes are in order, would be a ukulele with low g (GCEA). The reentrant tuning is what really give the ukulele it's distinctive sound.