Damage meter

From WoWWiki

A damage meter is an add on which calculates the damage done over some amount of time: anything from per second to per instance. Since many advanced meters also include options to measure other stats (threat, heals per time, etc.) they can be useful tools in evaluating characters since they take into account an individual's playing style rather than just mechanical stats.

They may also be used in parties, especially raids, to determine if players are underperforming to the point they are causing issues to overall progress; a group may then choose to pass coverted items to an otherwise decent player or kick a otherwise discretely lazy player tagging along for easy loot.

Nevertheless, over-reliance on damage meters as a measure of progress is controversial, especially in group play. It is generally good etiquette to not spam meter output into party chat unless asked to, as it may create the worry DPSers are solely worried about their damage output rather than the welfare of the group, the nature of specific battles, or any alternate roles they are expected to perform. This opinion is especially strong among tanks and healers, who are generally only concerned with DPS inasmuch as the amount of threat it produces.