Multiing (or WA multiing) is the practice of having more than one NationStates nation per player (or per computer) as a member of the World Assembly at one time. Having multis is a seriously illegal action and may result in all multi nations being ejected from the WA, or even deleted.
Controlling WA membership has been a long-time problem in NationStates. The premise is that a given player should have one, and only one vote or voice in the game. Given the importance of WA membership to the practice of invading a region, having more than one WA nation at a time would give a single player disproportionate power in the game. Thus, the rule has always been, one player, one WA nation.
Multiing also has an effect on the WA itself. A multi may endorse his WA puppet to grant himself the right to submit or approve proposals and repeals. Needless to say, that player would also have disproportionate voting power in the WA, but that tends to disappear in the enormous voting pool that comprises the World Assembly at large.
Because there is no way to tell how many players use a computer this is often stated as one computer, one WA nation. However, using more than one computer to have multiple nations in the WA is still considered both unethical and illegal. There is no limitation on the use of puppet nations, but only one may be in the WA at a given time.
Note that the moderators have more than one way to catch multis, and that a player cannot sneak around the prohibition by having different email addresses on each nation. There are regular arguments from family members who share a single computer, but because those are difficult to impossible for the game administrators to verify, the rule requires that such cases must still abide by the one/one rule.
Various terms are used interchangeably for this practice, including multiple, multi, multy, and multying.