What makes an Orchid an Orchid

[RosieC]

Ok, this is a question that friends keep asking when I say I have this new Orchid hobby and I don't have a clue what the answer is.

It seems like a fundamental question so I feel embarised that I don't know

What makes an Orchid an Orchid?

For example why is a Plurothallis an Orchid but a Dandelion is not?

What is the common factor that links them all together

With many of the genera I can look at the flowers and say "that's an orchid". Recently on a visit to a botanical gardens and while in the 'orchid room' I looked at some of the things my husband pointed out and said "No way is that an orchid, looks more like a bromiliad" and sure enough we had that confirmed.

But I can't do that with all the Genera and I don't even know what it is about them that I am identifying even when I can spot them.

So... does any one know the proper definition of what links them together. I would love to know.