Pixel buffer or pBuffer is a feature in OpenGL and OpenGL ES platform interfaces which allows to do off-screen rendering. It is specified as an extension to WGL API, and a core feature of GLX & EGL.
When using pBuffers user can bind OpenGL context to offscreen surfaces, effectively making rendering off-screen to default framebuffer allocated by OpenGL itself.
The pBuffer functionality has been later superseded by the usage of FBOs (Framebuffer Objects). However, it can be still used on modern OpenGL drivers.
pBuffers should not be confused with Pixel buffer objects (also named PBOs), which are not-renderable buffers containing raw pixel data.
References
Pixel buffer Wikipedia(Text) CC BY-SA