chat pose
A transparent image intended for use as an avatar pose in a 2D graphic avatar chat such as Chatlands (i.e. Wolfhome, etc.)
These images often are low-res and sometimes use aliasing, but are highly detailed, and are intended to be flipped to face the other direction. While most images of this nature uploaded to e621 will be commissions intended for the character owner's use only, users also sell (and distribute for free) premade greyscale poses.
They have become less common in recent years, as avatar chat sites have fallen out of use since their peak of popularity in the 2010s.
The poses in these images are intended to work with a variety of chatroom backgrounds, making the character look as if they are actually present in said location. Some common poses include an "enter" pose used for entering a chatroom (which typically depicts a character walking in), a lay pose, an "AFK" pose (often holding a sign with "AFK" written), among others.