what makes an orchid an orchid?
 

just curious, what is the defining characteristic that all orchids have, what is the one thing that makes them orchids. There r thousands of species of orchids and im just curious what it is that they all have in common?

I believe it is the fused male and female reproductive organs, along with the labellum ?
I'm sure someone will correct me if that's not right lol

I would say four things (generally speaking):
(1) They always have one petal that is modified into what we commonly refer to as the lip.
(2) The sexual organs of the plant are fused into a single structure called a column (this is a very general statement that really needs some qualification).
(3) The seeds are very small and without a food reserve.
(4) The pollen are found in "bundles" (pollinia) attached to a "thread" (caudicle or stipe).

A fifth item, orchid flowers are bilaterally symmetrical (also known as zygomorphic). A line drawn from the tip of the dorsal sepal, thru the column to the tip of the lip divides the flower into two mirror images.

To complicate things, there are exceptions to these rules:
(1) most Thelymitra
(2) Apostasioideae (if you consider these orchids)
(3) Disa uniflora, Bletilla striata
(4) Apostasioideae

Other traits like bisymmetrical flowers (Antirrhinum), having 3 petals and 2 sepals (lilium) and inferior flowers (Fuchsia) are also not specific to orchids.

Dont for get that orchid flowers are zygomorphic which means that the can only be divided equally on one plane to be mirror images of each other

An important aspect is that they have 3 petals and 3 sepals. Sepals can often be fused, as in masdevallia or paphiopedilum and one of the petals is usually in the form of a lip or pouch. They also usually (always?) have a column that forms the sexual parts of the flower.

As you've gotten many answers already. I want to point out a train of thought that may be getting in the way of your understanding of what makes an orchid, an orchid.

You're asking questions such as "what is the defining characteristic that all orchids have" and "what is the one thing that makes them orchids". This implies you're looking for just one common link.

From what you can see, there is no "one thing". Rather, I challenge you to think about it this way. It is a series of combinations of the many traits that may or may not be unique to the plants in the orchid family that many species of orchids share with one another.

Let me demonstrate...

1. 3 petals, 3 sepals - this trait is not unique to Orchidaceae.

2. A modified petal that is called a lip (labellum). Again not unique to Orchidaceae.

- But as was pointed out, some groups of orchids don't have a labellum.

3. Inferior ovary (not unique).

The list goes on...

But there are a few characteristics that are unique, such as:

1. In the many species of orchids that have flowers that are monoecious (flowers have both male and female parts), the male and female parts are fused into a column.

- But there are a few species of orchids that are dioecious (flowers that are either one sex or another - that one flower is either a male or a female, not both), such as those that belong to the genus Catesetum or Cycnoches. So there can't be a fusion of male and female parts in the column with these guys because the sexes are separate.

2. Photosynthetic roots in the epiphytic species of orchids.

- Meaning there are those that have non-photosynthetic roots, primarily those of terrestrial orchids.

Get my point? It's not one, but rather a good number of a combination of enough traits that many species of orchids share that gets them classified as an orchid.

If there's one thing that evolution has taught us it's that classifying plants and animals into different discrete groups with definable boundaries is meaningless. The boundaries between Families, Genus, Species, etc. are usually 'fuzzy', with borderline members that share properties relevant to both sides.

Classification of discrete Things or Groups with definable Properties is just a way of us humans better getting our heads around things and communicating with eachother, but it doesn't represent the reality in a meaningful way.

So i think ronaldhanko gave an excellent list of four things that are generally true of orchids, and Andrew made an excellent point that there isn't one thing that is true of all orchids (while simultaneously excluding all other plants)

Is there any plants that might be an orchid but not classified as such?

and any orchid that raises doubts and might not be an orchid?