Different names for video resp. audio filters There are many pairs of video and audio filters for the same purpose - one for video streams and the other for audio streams.

For example fade - afade, trim - atrim, split - asplit, loop - aloop.

The names for audio filters have the a prefix.

Why? Why they are different names for video and audio filters? 

After all, every filter is applied to a particular individual stream, isn't it? So FFmpeg knows in advance the type of that stream, and is able to decide itself what type of filter have to use (video or audio), even if the names would be the same.
Mainly because this is how things got implemented / evolved from the start. Yes, it could be a single filter but that would need major changes to the current code.