Nice job Adam, though I'd suggest it might be a good idea to limit the list to freediving-specific terms only, as many of the words on the list are already thoroughly defined by dictionaries. In those cases, the best you could do would be simply to cut-and-paste from a reliable source, trimming off the irrelevant bits to suit and letting freedivers add freediving-specific detail as required. Entering brief explanations in an unregulated, vernacular way may seem helpful to some readers but is a little misleading, as it suggests that this brief gloss is the true definition. It also leaves the door more open for inaccuracy to creep in, especially when it's a wiki.
Couple of examples:
"FAILURE DEPTH - The depth at which lung pressure becomes negative and therefore it becomes difficult, if not impossible, to equalize."
This belongs on the list but is not accurate and really needs to be quite specific, e.g. "May refer to either a theoretical (calculated) or practical (tested) depth, typically measured in salt water. Theoretical: the depth at which a diver's lung volume is reduced to Residual Volume (RV) assuming the diver begins at Total Lung Capacity (TLC) at the surface before descending and does not exhale. When RV is reached no air can be retrieved from the lungs by means of a 'normal' exhalation. For this reason Failure Depth often coincides with inability to equalise. Practical: the depth at which a particular diver typically cannot equalise. While practical Failure Depth may be partly determined by the diver's theoretical Failure Depth, additional factors such as equalising skill and gas exchange are usually involved... ..."
Or something similar, that's just an example rather than a detailed suggestion and is not complete. It hardly even starts to detail the alternative usages and definitions of the term. You can see though, that many of freediving's terms would demand a pretty rigorous definition if you wanted to be accurate and informative.
"FATIGUE - Complaints of being tired, experiencing a lack of sleep or a generalized tiredness."
Probably doesn't belong on the list but if you want to include it you probably need to take a proper definition from a dictionary and supplement it with detail relevant to a freediving context. The above definition is both incomplete and inaccurate (fatigue does not mean the action of complaining about tiredness)
Cheers,
Dave