There is no accepted definition of "idealism". Rather there are a family of philosophial views associated with the term "idealism", which have their historical precedent in philosphers like George Berkeley, Immanuael Kant, GWF Hegel, and many others. As a first approximation, idealists hold that the world is in some sense derivative on the mental, or that the mental has a metaphysically special place in the world. But these are merely pointers in a certain direction, no particular definition of "idealism" is agreed upon in the present debate. Nonetheless, idealism contrast with the now standard view that the mental is merely the result of an accidental arrangement of matter, which a nice bonus to reality, but not a central part of it. Idealism gives the mental a more central place in the world, somehow.
Similarly, idealism can be seen as denying that reality is completely mind independent, but to spell that idea out more precisely is a substantial task that can be attempted in many different ways, leading to many different forms of idealism.
Idealism thus gives a special place to the mental in the world. The mental here can include various parts or aspects of minds, like consciousness, conceptual thought, language, perception, emotion, and so on. Differnet idealists will stress the significance of different aspects of minds, again leading to various different forms of idealism. The members of the idealism network defend a diverse group of idealist views, with no consensus on which position is best nor on how idealism should be stated more precisely.