If you are talking about management less leadership then it is science. If you are considering leadership as well then it is Art.
Because science identify and quantify things through variable and constant. If one keeps these variable and constant at certain value, one will get the same results every time. Such as Toyota Camry 2008 model will be identical to other Camry 2008 model, exactly. Any up or down model will fail at QC.
In management you can emulate any thing as long as you deal with the things and follow the standard procedures. So if a man working on a machine, he should have exactly same experience every time. (No wonder people who work on assembly line feel bored and unchallenged.)
It is different when you deal with people. It is art and called Leadership. Because human can not identify or quantify through variable and constant. What is today contact will or may become variable tomorrow or vise versa. This phenomenon is also known as human factor.
Human factor is unwelcome in today's most of work places. We try to kill human factor through work training as much as we can. Why? because then workers will behave more like a thing than human. This will bring them into science part of management circle which is easy to create and easy to follow. (The biggest killer probably by this standard is our armed forces where every one is trained to almost the extent of brained washed.)