It's definitely a fern. Whether or not it's a weed depends on whether or not you like it where it is.

Some can be both, if you don't want a patch of ferns.

It's definitely a fern. The previous owners of our house planted them all over the yard and I'm constantly digging them up and giving them away or throwing them out. So to me, they're a weed... but it's a fern :)

I love ferns - they are beautiful, get no pests or diseases, grow in places where nothing else will grow and provide lush green foil for more colourful plants. How could you hate this?

Thanks to everyone for their replies! Will likely keep it unless it goes nuts.

That's lovely, Flora! They all look pretty much alike to me though I know there are many different ferns. My Mom planted some kind of ferns in her yard last year and the other day I noticed there's a much expanded circle of little fronds coming up. If it keeps doing that... it won't stop, right? Should I take some pics for here? I'd like to move it ASAP if it's a 'thuggy fern.' I know nothing about them, is there such a thing? There are other spots where it would be welcome to take over, but not where it is.

I am not a fern expert - I just love the look of all of them. Can't say I know of any 'thuggy' ferns. The ones I have never expand very far. Some are evergreen and stand all through the winter.This one is the evergreen Polystichum setiferum and it provides a good backdrop to other brighter plants.

That's very pretty, and TYVM! I've made myself nervous though. I'll take some pics of that spreading fern while we still have a chance to easily move it. I've not heard of thuggy ferns either, but don't usually enter their realm at all.

Can anyone tell me what this is

yarrow

Agreed, should smell like a sweaty shoe.

Oh, there are some seriously "thuggy" ferns although how common they are in the UK, I'm not sure. Our native bracken fern, Pteridium aquilinum, is a serious pest here once established as they travel by underground rhizomes as well as spores. I think this is a fairly cosmopolitan species, so no doubt occurs throughout Europe and Britain as well. Generally not often present in cultivated gardens but pops up regularly along side more rural roads and in any woodland setting.

Our native Bracken is also Pteridium aquilinum but it isn't really a garden problem except in country areas where there is acidic soil and it creeps in from the surrounding land. Otherwise all our ferns are benign.