A pose shaped like the capital letter "A", where the arms are held straight out and angled downwards from the sides of the body.
This is more relaxed than a T-pose or outstretched arms, but less relaxed than arms at sides where the arms simply hang against the torso due to gravity.
You often see this pose in a few contexts, for example:
A character showing off an outfit.
A character posed similarly to a Sailor Moon magical-girl style transformation scene or the like where the character appears to be relaxed, but floating.
The name comes from 3D modelling as an alternative to the more common T-pose. The A-pose allows for the shoulders to be modeled and textured in a more natural position, and the defined angle can aid in rigging.
