Sheikh means an elder, and its usage is far broader than any single office, which is precisely the point to grasp about the word.
One word, many roles
The title is applied to tribal leaders and to the rulers of the Gulf states, to men of religious learning and standing, to the heads of Sufi orders, and to the elderly and respected generally, so that it names a standing of authority or reverence rather than one defined office. In its religious use it commonly denotes a scholar of Islam recognised for his learning, or the master of a Sufi order who initiates and guides disciples, and in these senses it carries real religious weight, but the same word in another context names a secular ruler or a tribal chief.
Standing, not ordination
In every use the title marks recognised standing, whether of age, of lineage, of learning, or of spiritual authority, rather than an office conferred by ordination, since Sunni Islam has no ordained clergy, and it is bestowed by acceptance and reputation. To read the word rightly one must attend to the context, for the religious scholar, the Sufi master, the tribal chief and the Gulf ruler are all called sheikh, and the honour the word carries is common to them while the offices are entirely different. This breadth is characteristic of a tradition in which authority is recognised rather than conferred, and in which a single term of respect can span the sacred and the secular.