What does the HTML acronym "span" stands for?

It's not an acronym, it simply "spans" the content that is within the tag.

Pretty hard to find out what was going on in the minds of the creators of HTML4, where SPAN first appeared. I tracked down the original draft of the standard, but there is nothing said about the naming, only a definition:

SPAN is an inline element and can be used within paragraphs, list items, etc. when you want assign class or language information to a group of words. SPAN cannot be used to group block-level elements. SPAN has no inherent effect on rendering until you apply a style, e.g., via a style attribute, or a linked style sheet.

It's probably just derived from the verb 'to span'. But I would like to have found a reference:)

Update
Also from the HTML4 specification (the newest one this time):

Each version of HTML has attempted to reflect greater consensus among industry players so that the investment made by content providers will not be wasted and that their documents will not become unreadable in a short period of time.

Therefore, the SPAN element was probably first supported by a certain browser, after which it was taken over by other browsers and eventually adopted as a standard. So 10 points to the one who can name that browser and produce 199x IRC chat logs of that browser´s devs:)