Here's where I got to (7,100+ characters - to give an idea of the size problem, the character count increased by ~1,100 just with last-minute additions and tweaks):
Additive
Something that modifies the properties of a paint but without adding binder, e.g. flow improver, Tamiya Flat Base (X-21), retarder. See also medium.
Airbrush
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Binder
The adhesive component in paint. In hobby colours this is usually a vinyl or acrylic polymer, or a combination of the two, in oil paints it is typically linseed oil. See also medium.
Brush
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Catchlight
Another term for specular highlight.
Chalky paint
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Compressor
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Diluent
Anything that acts to dilute paint, e.g. water, paint thinner, magic wash.
Feathering
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Brushing out the edge of a layer of paint, e.g. to prevent a tide mark.
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Filter
See glaze.
Flow improver/flow release
An additive to reduce surface tension and improve flow. Can be useful when doing freehand as well as blending or glazing.
Freehand
In miniature painting, any patterns or decoration that are painted by hand.
Future
An SC Johnson product originally intended for use on floors. Used in miniature painting as a gloss varnish, to prepare a painted surface for decals and as the main ingredient in magic wash. See also Klear. Renamed in 2007 and currently called Pledge with Future Shine.
Glaze
Very dilute paint applied sparingly to the painted surface, with a damp brush. Sometimes referred to as a filter. Paint can be diluted with water, medium or a homemade mixture such as magic wash.
Can be used to tint or as part of layering to achieve smooth colour transitions.
Glazing
Using a glaze.
Gradient
A smooth transition from one colour to the next. Usually done by layering but also by wet-blending and airbrushing.
Hairspray technique
A weathering method usually used to realistically recreate flaking paint on vehicles. Hairspray is applied on top of a base colour and then overpainted with a contrasting colour (generally sprayed on, not applied by brush), then the surface is dampened and worked over with a brush or other tools. See also salt weathering and masking.
Highlighting
Applying paint of a lighter colour over the basecoat to simulate the effect of light. Can be done by layering or drybrushing. See also prehighlighting, shading, zenithal spraying.
Klear
Another name for Future in the UK. Currently sold as Pledge Multi-Surface Wax.
Layering
A painting technique where you apply many thin layers of paint, usually heavily diluted. Generally used to achieve smooth colour transitions when highlighting and shading.
Lining
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A painting technique where dark lines (often black) are used to outline areas on a miniature.
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Magic wash
A homemade mixture of Future and various other ingredients used to dilute paint for a wash, to help it settle into recesses and to prevent tide marks. Can also be used as a painting medium.
Mask
A removable protective layer that shields any part of a model from the next layer or layers of paint. When the mask is removed it reveals the colour (or the unpainted surface) underneath. Mainly used when airbrushing, to prevent overspray.
Masking
Using a mask.
Midtone
The middle colour value between the highlights and shadows. Often the same colour as the basecoat.
Mini
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Short for miniature, but specifically used to describe small-scale miniatures (below 1/35th scale).
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NMM
Non-metallic metals. A painted simulation of a metal surface. See also metallic paint, SENMM.
Open time
The workable period of a paint before it dries. See also retarder.
OSL
Object-source lighting. A painted simulation of light and shade on the surface of a model from a specific lightsource, e.g. a torch, campfire, muzzle blast or explosion. See also zenithal shading.
Overpainting
Painting on top of an existing area of paintwork. See also stripping.
Overspray
When airbrushing or applying paint from a spraycan, unwanted colour that falls outside of where it's wanted. Usually prevented by masking.
Paintjob
The completed paintwork on a model.
Preshading
A painting technique, usually used when airbrushing, where recesses are painted a dark colour before the basecoat is applied. Used instead of or in addition to a wash, e.g. for recessed panel lines on vehicles.
Prehighlighting
A lighter colour, usually white, applied over the primer to establish the basic shading. Can be done with an airbrush or with paint or primer from a spraycan.
Repaint
Painting over an existing paintjob, usually commercial pre-painted models. See also overpainting.
Retarder
An additive that increases open time (slows drying). A medium can have a retarding action, e.g. glazing medium.
Scumbling
A little like drybrushing but using a damper brush, with a scrubbing action rather than a light flicking motion. Usually done with lighter paint over darker paint. Can be used to simulate texture or on a flat or featureless surface to help it look less boring or uninteresting, by creating irregular colour variations. See also stippling.
SENMM
Sky-earth non-metallic metals. A painted simulation of a polished metal surface where there are clear reflections of the sky and ground, e.g. as you would see on a chrome bumper. See also metallic paint, NMM.
Shading
The simulation of light and shade in painting. Sometimes refers to just the painted shadows, which are usually added over the basecoat. Can be done by layering or with a wash. See also highlighting, zenithal spraying.
Specular highlight
The bright, sharp-edged, highlight on a reflective surface. In miniature painting most often used on gemstones and when metals are done using NMM or SENMM. Also called a specular or catchlight.
Sponging
A painting technique using small pieces of sponge or foam to create random patterns. Often used to simulate chipped paint.
Stippling
A painting technique where the brush is used in a jabbing motion, usually using a stiffer brush or one with the bristles cut down. Can be used to simulate texture or on a flat or featureless surface to help it look less boring or uninteresting, by creating irregular colour variations. See also scumbling.
Stripping
Removing paint from the surface of a model, e.g. using paint stripper, oven cleaner, acetone, Dettol. Often used as a last resort when a paintjob doesn't work as intended instead of overpainting.
Thinner/thinners
See diluent.
Tide mark
An unwanted line formed by paint drying at the edge of a coat of paint, generally seen with washes but sometime with glazes. See also magic wash.
Tint
To slightly colour an area by glazing with a thin, transparent coat of another colour.
Also, any mixture of white with another paint (although generally used to describe colours where the addition of white is noticeable, as in pastel shades).
Wash
Very dilute paint applied generously to the surface, where it settles into sculpted detail. Used as a simple method of shading.
Zenithal shading
A shading technique that simulates lighting from a specific direction (usually from directly overhead) rather than a generic light from above as more usually used in miniature painting. See also OSL, zenithal spraying.
Zenithal spraying
An airbrushing technique, usually where lighter colour is sprayed from above and darker colour from below over the basecoat, to shade and highlight a miniature. Can be used by itself or supplemented with brush-painted refinements. Usually combined with some form of masking. See also zenithal shading.
Einion
