What actually constitutes "expending a charge" ?

Many things (skills, etc) have cooldowns which can be circumvented by "expending" a specific type of charge.

Does this simply mean firing off the Discharge skill, or one of the other skills that clears our your charges when it occurs?

Or is there some other manner of selectively expending a charge? Does "charge expires over time" also count as "expending"? How about "lose all charges when" attributes on items? Does that count?

Sadly, I can find no definition of "expending a charge" anywhere in the Wiki, and so far, not in these forums.

Not 100% sure what you're asking, but based on what I think you're saying, "expending a charge" is a local mod on a skill gem. That is, when you read Flicker Strike's description, you won't be able to remove its cooldown by expending a charge with Discharge. You can only bypass its cooldown by using Flicker Strike again with a Frenzy Charge available, which expends the charge automatically.

Examples:
Situation A
SECOND 1 - 3 Frenzy Charges
SECOND 2 - Flicker Strike used. - 3 Frenzy Charges
SECOND 3 - Flicker Strike used. - 2 Frenzy Charges

Situation B
SECOND 1 - 3 Frenzy Charges
SECOND 2 - Flicker Strike used. - 3 Frenzy Charges
SECOND 2.5 - Discharge used. - 0 Frenzy Charges
SECOND 3 - Cannot Flicker Strike. Flicker Strike is still on cooldown.

Okay, totally not what I thought, but it sounds right. So you are saying: if a skill says "The cooldown can be bypassed by expending a Frenzy Charge.", that means that if a Frenzy charge is available when you execute the skill, that charge will be removed, and the skill will become immediately available again?

If so, that explanation should really appear somewhere in the wiki (if not the game). In fact, GGG should really state it differently; because the way it is stated now makes "expending a charge" sound volountary, like a choice.

GGG - "The cooldown will be bypassed if a Frenzy Charge is available, and that charge will be consumed."

adghar, thanks for the explanation. I have played for two years without realizing that!