I've been surfing the net and coming up with so many varying definitions. Some have photos, some don't. Some are mock movie posters. Some are plot synopses. I'm finding no agreement at all on the definition of a one-sheet.
In Hollywoodland, everything is defined by who's asking for it. Are you talking about a "one-sheet" for a screenplay? For a television series? Or what?
We use "one-page" at my company but, depending on whether it's for a feature or a television series, it might be also called a "sell-piece", a "one-sheet" or a "pitch-page".
Whichever it is, it really needs to be
just one page. This all came about in years past when execs wouldn't physically touch the page presented at a pre-sale story meeting. It was an assistant's job to place the page on the table so that they could read it but not touch it. There was some legal thing about them picking it up, or "taking possession" of it... but since then the one page has evolved into a convenient way of presenting material. No one wants to sit in a meeting and try to read multiple pages.
So, if it's a screenplay:
Logline
Synopsis
Depending on who wants it, you might need to put in a location breakdown, preliminary budget estimate, and note any attachments, such as actors or a director.
If it's a TV series:
The Concept
The Series (which is a lot like a synopsis)
Episodes (I always list the first 6)
Depending on who wants it, you might need to identify the audience and/or list your credits.
The best way to give the requestor exactly what they want is to ask for an example from them... then shape yours to what they are looking for.
Of course, if it's for a film, then, yes, some advertising art, like a poster, would apply.
Hope that helps.