I needed two terms to differentiate incoming and outgoing.  The terms are used consistently based on whether it's a Customer Invoice or Vendor Bill.  This is well documented, too.

Are there places in the UI where it says "Invoice" where it should say "Bill"?

I'll also point out that non-native English speakers can run GnuCash in their native tongue and then it's up to the translator to convert "Invoice" and "Bill" to the native words.

If you can point out places where "Invoice" is used instead of "Bill" I can fix that.  Otherwise, if I don't hear back about specific instances of misuse I will close this bug in two weeks.

I'd like to explain the situation for the different languages and translations. It turns out that in many languages (including German), no two words exist for the distinction between "bill" and "invoice". So for all these languages, we have one translation for both words if there is only one word, and a multi-word translation if there is enough space (e.g. "incoming invoice" vs. "outgoing invoice" or "Customer invoice" vs. "Vendor invoice"). I'll attach the result of 

  grep -EA 1 '("Bill")|("Invoice")$' *.po

and for those 22 languages where I could observe a translation of both words, it turns out only 8 actually have two translations whereas 14 (!) have only one translation for both words. You can verify this yourself from the attached file by 

  grep msgstr bill-invoice.txt | uniq -c | cut -d. -f1 | uniq -f 1

So from an internationalisation point of view, it would actually be better to get away from this "english peculiarity" of having two words, and instead use only one word but explain it properly by "incoming" vs. "outgoing". Of course, the final decision is up to the one who programs this part of gnucash, which means warlord is free to decide this for himself.

Created attachment 70920 [details]
Search result of grepping for "bill" and "invoice" in all po files

This cuts both ways. There's no need to change the English UI because of a '"<insert language> peculiarity" of having [only one] word'. It seems to me that one should translate "Bill"/"Invoice" to the equivalent of "Incoming Invoice"/"Outgoing Invoice" if the language doesn't have two separate words. Why can't this be handled by the translators?

Yeah, this is a translation issue.  If your translation is broken or confusing you should talk to your translator to get them to fix it.

GnuCash bug tracking has moved to a new Bugzilla host. This bug has been copied to https://bugs.gnucash.org/show_bug.cgi?id=351297. Please update any external references or bookmarks.