This is a good working definition for Intelligent Design via New World Encyclopedia:
Intelligent design (ID) is the view that it is possible to infer from empirical evidence that “certain features of the universe and of living things are best explained by an intelligent cause, not an undirected process such as natural selection” [1] Intelligent design cannot be inferred from complexity alone, since complex patterns often happen by chance. ID focuses on just those sorts of complex patterns that in human experience are produced by a mind that conceives and executes a plan. According to adherents, intelligent design can be detected in the natural laws and structure of the cosmos; it also can be detected in at least some features of living things.
Greater clarity on the topic may be gained from a discussion of what ID is not considered to be by its leading theorists. Intelligent design generally is not defined the same as creationism, with proponents maintaining that ID relies on scientific evidence rather than on Scripture or religious doctrines. ID makes no claims about biblical chronology, and technically a person does not have to believe in God to infer intelligent design in nature. As a theory, ID also does not specify the identity or nature of the designer, so it is not the same as natural theology, which reasons from nature to the existence and attributes of God. ID does not claim that all species of living things were created in their present forms, and it does not claim to provide a complete account of the history of the universe or of living things.
ID also is not considered by its theorists to be an “argument from ignorance”; that is, intelligent design is not to be inferred simply on the basis that the cause of something is unknown (any more than a person accused of willful intent can be convicted without evidence). According to various adherents, ID does not claim that design must be optimal; something may be intelligently designed even if it is flawed (as are many objects made by humans).
ID may be considered to consist only of the minimal assertion that it is possible to infer from empirical evidence that some features of the natural world are best explained by an intelligent agent. It conflicts with views claiming that there is no real design in the cosmos (e.g., materialistic philosophy) or in living things (e.g., Darwinian evolution) or that design, though real, is undetectable (e.g., some forms of theistic evolution). Because of such conflicts, ID has generated considerable controversy.
[1] Discovery Institute, Center for Science and Culture, Questions about Intelligent Design: What is the theory of intelligent design? Retrieved March 18, 2007.