The definition of a skeptic as we know it is one who doubts the truth of something. This meaning is similar in philosophy, but it is more specific. "Skepticism in Western philosophy is the attitude of doubting knowledge claims set forth in various areas. Skeptics have challenged the adequacy or reliability of these claims by asking what principles they are based upon or what they actually establish. They have questioned whether some such claims really are, as alleged, indubitable or necessarily true, and they have challenged the purported rational grounds of accepted assumptions." (Popkin 1) When skepticism is used as a regular word and not with a philosophical purpose, it means that on a daily basis, almost everybody doubts and is skeptical about some issues and claims. Whereas when referring to philosophical skepticism, it means to doubt "the possibility of any knowledge beyond that of the contents of directly felt experience" (Popkin 1).
Many philosophers are known to be skeptics. "The Eleatic philosophers rejected the existence of plurality and change, conceiving of reality as a static one, and they denied that reality could be described in terms of the categories of ordinary experience. Heracleitus and his pupil Cratylus thought that the world was in such a state of flux that no permanent, unchangeable truth about it could be found; and Xenophanes, a wandering poet and philosopher, doubted whether humans could distinguish true from false knowledge" (Popkin 1). When speaking of the Eleatic philosophers, Heracleitus and his pupil Cratylus, and Xenophanes; we are speaking about ancient skepticism, where "skeptical philosophical attitudes began to appear in ancient Greece about the 5th century BCE" (Popkin 1).
Other philosophers (or sophists) that are more familiar to our basic learning of classical philosophy that were skeptics in their time were Protagoras and Descartes.