The leader or head admin of a wiki is a person who is the principal, senior administrator in charge of the site; either by means of founding the wiki, being asked to take over from the retiring founder, or adopting a dormant or abandoned wiki site.
The leader may be an active contributor and key member of the community of editors, or be completely hands-off.
Wikimedia Foundation leaders are usually members of both sysop and Bureaucrat user groups, which means they usually hold highest user group rights in order to enforce their leadership if necessary. Depending upon their level of wiki expertise, they may also be a member of the interwiki user group, or may even have created an additional bot account. But this is only one possibility of many. High user group rights are no prerequisite for wiki leadership.
WikiWikiWeb, & early wiki
Wikis are a community effort. Flattening out the community - in their abilities to build the wiki - is an important aspect of the software of most early wiki.
The number of leaders of any wiki can vary. On a Bliki, because it is personal, and not a collaboratory effort, there will be just the one leader. Depending on the type of hierarchy, and size of a wiki, several leaders may be required. For a wiki about grassroots movements and grassroots democracy, the hierarchy would be as flat as can be, which makes every active member of such community a leader. This depends very much on the founder or group of founders, and their approach. Either the focus lies on the wiki as a structure, or on the wiki people as a community.
Today, excellent wiki engines are available for free; and using the resources of a web server, can easily be paid by advertisement. Therefore, everybody can start their own wiki for free and without real technical expertise in a minute or two on wiki farms such as Wikia or Orain. There is hardly any threshold to take. Therefore many wikis are 'founded', and many are failing as well. Our index is full of documents of failure, and our category:Dormant is better populated than anybody could wish. The corpses of big and small wikis are laying side by side in the Category:Dead and Category:Archived (which means someone lodged a snapshot of such wiki before it died). Good leadership is still rare. And wikis are like living entities. If folks turn their backs on them while they are young, they will die. And if you turn your back on them while they grow up, they will run away, [fork], deviate, get depressed, become illegal, crumble down or overthrow you. It is not enough to label somebody a leader. They also must actually be a leader, and do that work well and consistently. Wikis are micro-entities within the potentially hostile environment of the Wild Wild Web that can be spammed like a locust infestation, or disturbed by trolls. Therefore the work of a leader is not less complex only because a wiki is small, rather the opposite.