To the layman, these terms have no special significance, however to purchasers or those involved in the sales of jewellery or gemstones, they may have significant meaning.

NaturalA gem formed completely by nature without human intervention during the formation.
SyntheticA laboratory grown gem that essentially has the same chemical, physical, and optical    properties as the natural gem.
SimulantAny artificial or natural product that is used to imitate a particular gemstone. This simulant will have chemical, physical and optical properties unrelated to those of the natural stone.
Treated gemstonesGemstones that have undergone any human intervention other that cutting, polishing, cleaning, and setting with the intention to permanently or non-permanently change the appearance of the clarity and/or colour.
Heat TreatmentWhere a gemstone is subjected to heat to intentionally alter, deepen, or change the appearance of the gem either colour or clarity.
Fracture filled/Clarity enhancedWhere the surface reaching fractures of the gemstone have been filled with a substance so to improve the appearance of the gem. This is not seen as a stable process.
Flux HealingThe partial healing or closing of a fissure in the presence of molten flux during the heat treatment.
Glass filledWhere the surface reaching fractures of the gemstone have been filled with glass to improve the appearance of the gem.
OilingWhere gems are soaked in oil which penetrates the surface reaching fractures and enhances the appearance of the gem.
Diffusion TreatedChemicals and heat are used to induce colouration in a gemstone. It can be surface related colouration or colouration throughout the stone.
FluorescenceThe appearance of luminescence in a gemstone when viewed under ultraviolet light.

Laboratory grown diamonds- in fact a misnomer- they are grown in a factory under laboratory-like conditions.

HPHT (High pressure high temperature) diamond growth:The process whereby diamonds are grown in a laboratory-type environment, using both high pressure and high temperature in the formation process.
CVD (chemical vapour deposition) diamond growthThe process whereby a material is deposited from a gas onto a substrata and that chemical reactions are involved. The gases normally used are atomic hydrogen and methane. These gems are normally a shade of brown and the HPHT process described below is used to permanently alter or change the colour of the CVD grown diamond.
HPHT- process (High pressure high temperature)Where a diamond has been subjected to high pressure and heat to the point that the structure of the diamond is altered or affected with the intention of changing or altering the natural colour.