Off the top of my head

It is the expression of emotions through, static or in movement, using our bodies. Mime artist, for example, can simply change the expressed mood by a facial or a physical shift and we the audience apply our individual interpretation. Another dimension is the relationship that groups on stage when interacting can evoke an emotion that emendates from the shared collective cognitive emotional intent.

I am not defining music beyond the requirement that it is sound ordered by measures (rhythm).

I think the above will stand even if the dancer is responding to a rhythmic pattern in his/her own head, conscious or unconscious.

To comment: without a minimum such definition it would be difficult to differentiate Dance from some other discipline. In other words I am assuming that your question is not privately casual or flippant but rather in the long intellectual tradition which has already concluded that water stops being water when it is wood or plastic. A critical requirement in this tradition is that we do not make use of the term being defined in the definition of the term.

Philippa. The point of attempting to say what something is; is to define the thing/object or activity rather than the person performing. Hence although I can define a category which has the label Dance I am incapable of even beginning to define a category of Dancers; mainly dancers like are like scientists, politicians and so on. Does this bring clarity some clarity?