Exactly its a very loose term. even Aladdin Sane and Diamond Dogs could be considered exprimental.
And that is exactly what people tend to forget. Judge the records against their time and against what the artist has come up with before and you will find that most of his albums were experimental. Being experimental is not restristricted to producing incoherent noise combined with half-baked concept ideas - it is also the process of breaking with what was done before and with what is the common listening habit at the time of release. So in Bowie's case 'experimental' includes albums like "Ziggy Stardust", "Young Americans", "Let's Dance", the Tin Machine work and "Outside" to name but a few. If they were all good albums is a different matter.
And I want to believe In the madness that calls 'now'