On crisp autumn mornings we often see them covered in dew. Coming from South East England I have always called them cobwebs. Recently I increasingly hear them referred to as spider webs. I prefer to hear cobwebs rather than spider webs, mainly because that's what I'm used to, but also because why would one use two words when one word will suffice, and also there is no ambiguity around the plural of cobweb as there is with spider web! I'm interested to know why anyone would say 'spider web'. I'm aware some websites state that a spider web still has a spider in it, and a cobweb is dusty, but I've never heard that distinction made in reality. I'm interested to know people's individual ideas on this, so please don't just cut and paste from other sites to answer this. Is it just that the usage of 'spider web' is overall more prevalent in US English?
"a spider's web," early 14c., coppewebbe; the first element is Old English -coppe, in atorcoppe "spider," literally "poison-head" (see attercop). Cob as a stand-alone for "a spider" was an old word nearly dead even in dialects when J.R.R. Tolkien used it in "The Hobbit" (1937).
Figurative use for "something flimsy and easily broken through" is by 1570s. An old Norfolk term for a misty morning was cobweb-morning (1670s).
Many spiders build webs to catch insects to eat. "Spider web" is typically used to refer to a web that is apparently still in use (i.e. clean), whereas "cobweb" refers to abandoned (i.e. dusty) webs. This being said, In AmE the distinction is not enforced and any web in any condition in any place is usually referred to as a spider web.