Phantasms, Specters, Shades... Banshees? It might seem odd that one monster in the game has four different names. We first learn about Banshees on "They May Be Slow...". Hans, one of the dead Germans, warns us about them as soon as we meet him:

We just couldn't kill them; our bullets went
right through those phantasms! Watch out,
they're everywhere.

If you enquire about the "phantasms" Hans will reply:

Muller called them shades. There's this
horrible, chilling moment when they scream
and your vision flickers and you wonder if
you're going to die. We never found a way to
kill them.

So Muller called them "shades". Hans will provide more details about "shades" if you ask him:

He called them shades because you can see
through them, like they're just shadows.
They have two, glowing red eyes. Run as
fast as you can, if you see one.

Later in the conversation Hans will offer another name for "phantasms":

Our machetes worked great on the headless
creatures, and we saved bullets by killing
them that way, but blades didn't hurt the
specters.

So now we have a three names (Phantasm, Shade, or Specter) for the same monster and all given by the same person.

We meet a Phantasm, Shade, or Specter at the very beginning of the next level ("... But They're Hungry"). If it kills you you'll get this death message:

You have died. Those Banshees are really annoying,
aren't they?

So the death dialog offers a fourth name "Banshee". Confused? Why four different names for the same monster?

The term "Banshees" is never used in any of the Dead Conversations. It only ever appears in a death message.

The term "Phantasms" is actually used in the name of the next level "Evil Undead Phantasms Must Die!". This is where we meet (unsprisingly) a large number of "phantasms". Yet the dead German on this level (Claude) refers to them as "specters":

A specter got so close to me I could hardly
see. It was cold and dark, just like now. I
was already wounded ...

and...

The specters! Please leave, before they find me.

Not to be outdone Joachim on the next level (Ascension) reverts back to the name "phantasms":

I let one of those phantasms get too close to
me. That was nearly the end.

and...

Those invincible phantasms are finally what
convinced Muller we should turn back, I
think. There are at least three on this level,
you should turn back, too.

You'll also find the term "phantasm" used in the monster list (STR# 2001) of the Pathways Into Darkness (v2.0) resource file.