While writing an article for SearchServerVirtualiztaion on snapshots one of the editors posed the question. What does AVHD stand for?  Honestly, I didn’t know.  I knew a VHD stands for “Virtual Hard Disk”, but what about the “A?”  The customary search through popular search engines came up mostly empty except for one.  Actually four, but the all had a variation of the same title (search results) so I will call them one for the sake of argument.  Anyway, it wasn’t something that was easily found.  There were plenty of references of .AVHD files and what they are, snapshot differencing disks, but very limited reference to what the “A” represents.  So what did this one search reference say the “A” stood for?  “Automatic Virtual Hard Disk.”  Seems to fit, but I was still skeptical given the limited results to back it up.  This led me to pose the question to others in the Virtualization field.  Here is what they came up with:

Ben Armstrong (Virtual PC Guy) - “We have never (AFAIK) actually documented what the "A" is for.  But it is "automatic". “

John Howard - Hyper-V and virtualization blog  “Rob - yes, the A stands for Automatic. Under the covers, an AVHD is simply a differencing VHD with a different extension. We use the different extension to make it more obvious that users shouldn't be manipulating VHDs directly, rather use the snapshot capability directly.”

Mike Briggs - Senior Support Escalation Engineer (Microsoft System Center Support) “Although I haven't seen any official documentation anywhere that spells it out clearly, we have always referred to it as "automatic" which I think stems from Rob Larson's Resource Kit for Hyper-V. (Isbn# 978-0-7356-2517-4)”

A little more clarity on the subject.  Not sure if this makes you or me any smarter, but maybe there will be at least one more search result in the list.