How can one define "planing"?
Have to say… I was rather surprised to see a very long debate on BoatDesign.Net, re 'the definition of planing', so thought I might add a little historic background for interested readers.
As a naval architectural student in the 50s, I grew up with the understanding that it was simply "an expression of the effect of net positive hydrodynamic lift, that permitted a hull to be somewhat lifted and therefore lower total resistance and run faster". We used to talk of 'partial planing' or 'full planing'. That was certainly the framework of understanding when listening to provocative Uffa Fox in a tavern in Cowes, UK as a starry-eyed teenager! We always knew we were in the company of someone pretty special in the field, as Uffa had taken a development first created for wartime seaplanes at nearby Fairey Aviation and then applied it to sailing dinghies, to go down in UK history as the first to do so—initially for his International 14, Avenger.
He later became involved with Fairey Marine for the construction of 1000s of hot-moulded Fireflies, Albacores, and Jollyboats etc, to develop the first known 'mass production planing dinghies'. Around the same period, Lindsay Lord (USA) wrote his equally provocative book on 'The Naval Architecture of Planing 
Hulls', explaining some of the early work re 'what shapes planed best and how they were affected by width, bottom loading and bottom twist?' The latter could, and typically does, create both dynamic lift and negative suction, so that's why we always referred to planing as "the net dynamic lift". [Later, as a young man, I worked in the design office of a Canadian shipyard that built a series of Lindsay Lord's monohedron hulls for Venezuela and got to hear his theories first hand.] His book clearly explained what created the best planing surface and how this needed to be modified for sea-keeping—compared to the very wide, calm water hull shape that ideally had a totally flat bottom with an L/B of only about 1.5, in order to create the most lift from what was, a long leading edge (transverse in this case). With this width, the planing angle would only need to be 1–2° as a low angle also cut wave drag (or hydrostatic resistance). But as such a wide beam becomes increasingly resistant in any sort of rough water, it had to be reduced considerably to be practical, and with that, a greater planing angle became necessary to compensate for the loss of the long, efficient 'leading edge', The speed at which this would provide 'net dynamic lift', also then had to be pushed up—demanding more horsepower, or in the case of a sailboat, a boat that was 'sailed flat' as well as with more sail area OR, a more efficient use of the sail that was there. The high tensioned 'kicking strap' (to hold the boom down square) all came from this, so that the sail could be flatter, giving more effective surface, as well as adding extensively to both rig and boat control. [Uffa Fox's initial answer to 'sailing flat' was to add sliding seats to his boats—and in fact, he sailed his canoe, so rigged, across to France and back!]
The 'compromise' beam and trim for acceptable sea going, is now more typically with a L/B of about 4 or 5, with an accompanying planing angle of about 4–5°. [The L/B ratio seems to run pretty close to the required numeric 'planing angle' to compensate for the reduced beam, over the practical range normally considered—purely an interesting coincidence though.]
Today, a more recent reference could be the technical paper on 'Planing Hull Resistance by Kukner and Yasa', published by the Istanbul Technical University (Turkey), that quite reasonably expresses the planing situation like this:
"As the speed increases, the vertical pressure component [lift force] is increasing and [together with] the hydrostatic forces [buoyancy] will cause the vessel to be lifted out of the water. When hydrodynamic forces are dominant, these vessels are known as planing hulls (Blount & Fox, 1976), Hydrodynamic and hydrostatic pressure forces can vary and be subject to the Froude number. Generally, the planing regime starts for Froude number Fn > 1.2, with Fn = 1.0 being the lower limit (Faltinsen, 2005)".
This paper further indicates that, with the right hull form (ie; sufficient planing surface at acceptable surface loading), the hydrodynamic forces could provide 50% of the lift at around a speed/length ratio (V/L½) of 1.2 and become the total lift provider at 2 or 3 times that speed.
It's good to keep in mind that basic boat resistance is typically made up of 4 items; Frictional, wavemaking, appendages, and wind/air. (Added 'resistance due to rough water' would be a 5th when applicable.) Typically, the first two are the main ones, but that depends on the speed. [VERY high speeds are only achieved when the first two are very much lowered, and then, appendage and wind resistance could in fact be the only things left!—such as for an AC72 (America's Cup cat) while foiling.]
But ignoring appendages and wind for the moment, we're left with the very substantial basic resistances of wetted-surface friction and wave making. Fast boats HAVE to lower at least one of these in a major way. Power boats that are by design fairly heavy, do this by pushing up their hydrodynamic lift 'to plane', at least partially clear of the surface. Going 'too' extreme with this will put the boat airborne, with a potentially disastrous loss of control—so there's a limit here. Through design, it's not difficult to reduce hydrodynamic lift though and this will generally result in a more Veed hull that can take rough water better at speed.
The other approach is to reduce the wavemaking by making hulls lighter, smaller and much slimmer—so that they hardly create any waves at all. This is the way multihulls have succeeded to reach high speeds. The surface friction is still there and will then be the predominant factor, but the area can be slightly lowered due to smaller hulls, made possible by lighter materials. Overall, the slim, low wave-making hulls generally win out over the planing hull and the slim‑hulled multihull will also be less affected by rough water at high speed—so typically, multihulls can be designed to be faster than planing monohulls, assuming the same drive force. Ultimate stability of each type, is a whole other subject however—again with pros and cons, as the two types (monohull or multihull) are so different in basic qualities.
There is more on planing in the article "Do trimarans plane?"
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