Clairvoyance entails knowing about a person, place, thing, or event without using the ordinary five senses to gather that information. If you had the ability to do this, you could be called a clairvoyant. Clairvoyance is typically used to describe knowledge of the present, rather than that of the past or future.
Some explanations for purported instances of clairvoyance have been written off as simply being caused by memories of subtle legitimate sensory stimuli. For example, you are in a crowded room, and everyone around you is talking. A stranger comes up, and you correctly guess his name is, "Henry RandomName III." You could be said to be utilizing clairvoyance to gather that information, but you may have also overheard him introduce himself earlier. While that information might not have manifested itself consciously, (i.e., you didn't actually remember hearing his name being said,) it may have been absorbed subconsciously, and was later called upon and intuitively known by you.
There are also many instances in which no other explanation exists as to how certain pieces of information were gathered, and so clairvoyance is assumed.
Despite the potential for being connected, clairvoyance and telepathy can be easily distinguished, at least in definition. Clairvoyance differs from telepathy in that a telepath would gain information from the mind of another individual, whereas a clairvoyant gathers information from some other source, without the aid of mind-reading.
Some people believe clairvoyance, telepathy, retrocognition, precognition, speaking with the dead (mediumship), and all other abilities to gather information in a non-sensory way are all variations on a theme. That is, they all arise from a single, deeper process.
Clairvoyance has many branches of use and technique, one specific of which is remote viewing, the process used by the Stargate Program from the 1970's to the 1990's.
One common claim of an instance of clairvoyance, which has been made by many people for many years, is the event of seeing or imagining a loved one just after their death, but before learning about their having passed on.
Before his death, Abraham Lincoln is said to have dreamt he was at someone's funeral. As the dream progressed, he came to learn that the funeral was his own.
A rare, and yet far more powerful and demonstrative case of clairvoyance has occurred in more than one person. The person is usually not (apparently) above average intelligence. However, upon going to "sleep," and entering a sort of trance-like state, they are able to speak as if fully educated, and even diagnose diseases in others, while presenting cures that have been shown to be highly effective. Victor Race was able to achieve this sort of sleeping clairvoyance in the 1780's, and a more well-known individual by the name of Edgar Cayce made a career out of it in the 20th century.