An artifact simply means any product of human workmanship or any object modified by man. It is used to denote anything from a hammer to a computer system, but it is often used in the meaning "a tool" in HCI or Interaction Design terminology.
The term is also used to denote activities in a design process. For example, in Unified Process (an object oriented system development methodology) a "design artifact" is sometimes used to denote the outcome of a process acitivity such as use cases (Larman 1998).
The antonym of "artifact" is a "natural object" - an object not made by man (Wordnet, Princeton University).
Please Note: Artifact may be spelled with an 'e' instead of the 'i'; artefact. According to Google.com, artifact with an 'i' is the most common spelling of the word. Searching for 'artifact' returns app. 500.000 search results whereas 'artefact' returns 90.000