There are too many antonyms to count!
It depends on how you want to use the word: The cola is not flat, it's bubbly .
The tire is not flat, it's full .
Her lecture was not flat, it was exciting .
His humor isn't flat, it's wry .
The earth isn't flat, it's round .
Business is not flat, it's booming .
That roof is not flat, it's sloped .
Our piano is not flat, it's sharp .
His feet are not flat, they're arched .
The tax scale isn't flat, it's graduated .
His denial of the allegations were not flat, they were wavering .
You made the classic "mistake" of asking for an antonym without furnishing a context!
I'm guessing you had something more like this in mind:
The countryside isn't flat, it's hilly .
but that might not work near a valley.
ell.stackexchange.com/users/113/J.R.
If you're talking about the surface of a colony of fungus, and it is not flat, there are many different terms you could use based on the level of "not-flat-ness" (patent pending):
Rough - Think like the texture of sandpaper or jeans Ribbed - Like corduroy pants
Varied - Like rolling hills.
Areas are flat, and others sloped Sloped - Like a round hill, or the top of a mushroom Cracked - Has cracks in it Creviced - Like cracked, but deeper Undulating - Marked by steep rolling hills Pockmarked - Filled with small holes, like a sponge Fissured - Similar to cracked, but stronger Craggy - Like the rocky face of a cliff Cleft - Split in the middle, like a cleft chin Mountainous - Mountains
Severe - Steep Pitted - Like pockmarked, but
deeper holes Not flat - Exactly what it says, something's not flat.
They all have subtle meanings, implying different levels of "not-flat-ness." Also, as others have pointed out, you would not usually say that something is both flat AND rugged.
In English, rugged is typically used to amplify the adjective describing how unflat something is.
So mountains are rugged, but a flat plains land probably wouldn't necessarily be, unless you're trying to imply that at first glance the prairie looked flat and easy to cross but it turned out not to be because it was full of danger.
ell.stackexchange.com/users/10562/Maurice Reeves
I would go with: "Undulating".
ell.stackexchange.com/users/12603/Peter Crotty
The problem here is that asking for a generic antonym for "flat" is rather like asking for an antonym for "neutral" or "plain", or perhaps "silent" or "odorless" — they're all words that describe the absence of any features of a certain type, and so any informative antonym would need to describe which features are actually present.
Or, to misquote
Leo Tolstoy :
all flat surfaces are alike, but all non-flat ones are different.
That said, if you do really need a generic antonym for "flat", e.g. to present a binary choice between flat and non-flat, some possible choices could be: "textured" ( as suggested by BM- ): Tends to imply the presence of small-scale variation; sometimes used as an antonym of "smooth", or as a milder near-synonym for "rough".
Common in certain technical fields; for example, a graphic artist might describe the color of a surface as either flat/solid or textured. "varied" : A generic word for "not all the same".
May be used synonymously with "textured", but with a slightly broader sense.
"elevated" : An antonym of "flat" in a somewhat different sense than either "textured" or "varied".
Could be applicable if "flat" is being used in a sense of "lying close to a surface".
"not flat" :
This really is the most generic possible antonym for "flat".
In fact, many general English antonym pairs, like "colorful" / "colorless" or "visible" / "invisible", are of this type — the word "flat" just doesn't happen to have a regular suffix like "-ful" that could be inverted to "-less", and doesn't customarily accept a negative prefix like "un-" or "in-", so it has to be negated somewhat more awkwardly.
(That said, *"unflat", while not a commonly used word in contemporary English, would certainly be an understandable construction.
I'm not really advising you to use it, but you could do it and get the meaning across.
"Non-flat", as I've used earlier, would be a somewhat better compromise.)
ell.stackexchange.com/users/498/Ilmari Karonen
Possibly "textured" might work, if further description of the surface is not required.
ell.stackexchange.com/users/12555/BM-
The surface of the colony* is flat and rugged.  The surface of the colony* is  not flat  and rugged.   *(a colony of fungi)  Flat  is in this case refers to the elevation of the colony. From the side view, it's flat, like a computer disc.   What is the opposite of flat, for this context? Is it non-flat?
what is the opposite of the word 'flat'?
ell.stackexchange.com
<antonyms><negation>
ell.stackexchange.com/questions/41518
ell.stackexchange.com/users/12595/student
There are many antonyms for flat, including textured, hilly, varied, wavering, graduated, undulating, sloped, craggy, rugged, cracked, and creviced. Elevated, textured, undulating, hilly, sloped, rugged refer to terrain. Wavering, exciting, wry, round, booming, sharp, arched, graduated, bubbly and full are possible variants.
Elevated, textured, undulating, hilly, sloped, rugged refer to terrain. Wavering, exciting, wry, round, booming, sharp, arched, graduated, bubbly and full are possible variants.