I'm sure you've heard the expression "It's an Art, not a Science". You may have even used some version of it yourself. I think I hate that statement.
You may have been (or likely will one day) go to the hospital for an operation. And may expect to be put under a "general anaesthetic". But how does it work? You'll take something else to put you to sleep but how does this other drug stop you from feeling pain that would otherwise shock you right out of any deep sleep? They've been used in medicine for 150 years so this should be a simple question right?
"Advances in cell biology, genetics and molecular biology have transformed anesthesiology into an active area of research. Scientists have largely abandoned the idea that anesthetics work by acting on fatty molecules in cell membranes. The bulk of the evidence now supports the idea that the drugs target specific protein molecules embedded in nerve cell membranes and interfere with neurotransmission. Researchers now believe that each anesthetic acts on a different set of molecules to bring about its characteristic effects." - Anesthesia Fact Sheet
In other words: after nearly 200 years of research we still don't know exactly how they work. And yet it's used safely around the world on millions.
But it's still a Science. Why? Because it's cause and effects are very well researched and measured. We know exactly what kind of dosage will have what kind of effect. They "why" isn't critical to that understanding. While any profession may have a level of artistry no one would say an anesthesiologist is more artist than scientist.
So why do we use the term with business? Estimating, planning and building software all have known inputs and expected output. But why do we pretend there is no 'science' in these decisions and instead often rely on 'gut feeling' or personal experience.
As far as I can see if there is a reasonable way to measure it (ROI, time, cost, man-hours, bugs, client satisfaction, employee satisfaction or whatever) then it can be treated as a science even if there's a large margin of error. What works and what doesn't. What has side effects and what doesn't. Did the business change you enacted have the effect you wanted? Are you comparing it to another group (a control if you will) that hasn't gone through the same changes. What studies are there on previous groups going through similar changes?
Personal experience has value. But reproducible and measured results are science. The rest are BS artists.