Short answer: Skepticism is the practice of doubting claims that do not appear to be supported by current scientific knowledge.

Long answer:
Skepticism is an essential tool of science. In every field of science, practically every day, someone claims a new hypothesis to explain a phenomenon. Can every one of these theories be valid? Which ones have merit? Skepticism provides the tools to separate valid theories that explain reality from weird ideas with no connection to reality. Between these two extremes lies a spectrum of ideas that can be difficult to judge. Skepticism gives us objective tools to analyze claims about reality and determine where along the spectrum of plausibility they belong.

Carl Sagan famously summed up skepticism in a way that only he could: "Extraordinary claims require extraordinary evidence"

Note: Ottawa Skeptics promote the use of scientific skepticism, please do not think that we are supporters of philosophical skepticism, which is a very different idea.

Other definitions of skepticism:
Wikipedia
Skeptics Society
UK Skeptics