[FFmpeg-user] What is a "pad" in the context of an "input pad", an "output pad" and a "filter pad"

[Clay]

Dumb ffmpeg question alert:

What is a "pad" in the context of an "input pad", an "output pad" and a
"filter pad"?

I understand the concept of padding a video with horizontal or vertical
bars, padding audio with dead air or some other audio, but this is
different.
Is there a technical description of the actual term "pad" used to
describe command line items in the ffmpeg execution string?

There are a few references in the main ffmpeg documentation
(https://ffmpeg.org/documentation.html) and a search of the ffmpeg-user
archives brings many usages of the term, yet no actual description (385
hits:
https://www.mail-archive.com/search?l=ffmpeg-user%40ffmpeg.org&q=+pad+).

While I can infer the meaning, it would be great to get a meaningfully
pedantic description from someone intheknow.

On the interwebs I found this description:

PAD [https://www.computerhope.com/jargon/p/pad.htm]
Updated: 10/17/2017 by Computer Hope

PAD may refer to any of the following:
1. When referring to a network, PAD, short for packet
assembler/disassembler, is a device capable of converting from one
packet into another. 

...I presume this means to describe a device that converts packetized
data from one packet format into another packet format while preserving
the data [or information] contained within the original packet.
...This seems a little bit like converting a stream/file from one codec
(e.g. h.264) into another (e.g. h.265) but that doesn't quite match the
inference.