I’ve heard a few pipers refer to certain tunes as “single reels”. In this clip of Brian McNamara, he mentions single reels in his intro.
I’m familiar with the difference between single and double jigs, but what defines a “single reel” and how does it differ from a “double reel”.
Single reels have short parts (A, B, C etc.), typically half the length of “normal” reels. That is, 4 bars repeating or 8 bars non-repeating. As opposed to 8 bars repeating / 16 bars non-repeating. I suppose the latter are “double reels” by that terminology, but I don’t think I’ve heard that term used.
Melodically, to my ear single reels often seem rant-y, with a tendency to be more strongly repetitive and “patterned” with arpeggios and scales, and less melodic movement.
Examples:
The Heathery Breeze: https://thesession.org/tunes/411
The High Reel: https://thesession.org/tunes/44
The Galway Rambler: https://thesession.org/tunes/634
What Mr MTGuru Sir said. 
You quite often hear people referring to playing a reel either “singled” or “doubled”.
I guess I’d take that in terms of choosing to play a “normal” reel with the repeats (doubled) or without (singled). As opposed to a “true” single reel with the characteristics I mentioned above.
It seems to have become the name for tunes played with the parts unrepeated.
I don’t think I heard the terminology used at all before maybe the last decade. Some tunes you played ‘single’ but they weren’t called ‘single reels’. I sort of suspected fora while it’s one of those internet things. It is probably telling ‘the Companion..’ doesn’t have an entry for single reels (or mention of them under ‘reels’)
As for the very simple repetitive tunes as MT describes them, I sooner think of tunes like the Monaghan Twig or John Kelly’s concertina reel and tunes like that (the ones the old, often concertina, players used to belt out) in that context, than some of the examples given. But YMMV. Very simple tunes with a very strong and simple and repetitive rhythmic structure.
Of the examples MT gives, don’t think I have heard the High reel played ‘single’ and the Heather Breeze can go either way.
Interesting maybe in that context is the recording of Patrick Kelly talking about things like that and saying the old players always played tunes single, never repeating parts.
examples:
T:John Kelly’s concertina reel
M:4/4
L:1/8
R:Reel
K:DDor
AddB ABcB|AEGE DEFG|AddB ABcB|AEGE DEFG:|!
AddB cdec|dcAB cAAB|AddB cdec|dcAB c3 B|!
AddB cdec|dcAB cAAe|~f3 d ~e3 d|cAAG d2 :||!
T:John Naughton’s
M:4/4
L:1/8
R:Reel
K:D
B2 BA Beed|B2 AG FDFA|B2 BA Beed|B2 AG FDDA:||!
fede fd d2|fede fbba|fede fddB|ABde fdde|!
fede fd d2|fede fbba|fgef dedB|ABde fded||