When I type any word ending with an 'l' (ell, like "local") and then press the / key, the 'l' is replaced with another symbol and the slash, '/', is omitted. How can I deactivate that function?
This looks like the behavior of some very old Mac fonts (Palatino was one of them) where you could make the character l with stroke (used in Polish and elsewhere) by typing such a sequence. Try changing your font. If this is in fact the cause of the problem, you should get rid of the font that does this, which should not be on any modern mac.
Seeing that you use white on black, I take it you use the "Pro" profile (unless you duplicated it or created a fresh new one).
I would look into Terminal's preferences and check the used encoding and other options. If there are differences with, let's say the "Basic" profile, try to see if using that profile is behaving the same or behaves as you would expect. If it is as you expect it, see about changing your used profile more and more to the comparison.
I had this same problem because I had an old Monaco font in ~/Library/Fonts/. I was able to disable the old font with FontBook, and the issue was resolved.
Note that I had the same issue with a number of leftover old fonts. I was able to clean them all out in FontBook with "Look for enabled duplicates..."
This looks to me like a rendering bug rather than a feature. I would try increasing the font size on your terminal.