[SilasX]

Safe implies idempotent.

"Same effect from 0 or more actions" implies "Same effect from 1 or more actions".

[charonn0]

All safe methods are idempotent, but not all idempotent methods are safe (e.g. PUT or DELETE.)

[marcosdumay]

Safe means you can call it in any way you want. Like from a bot, or an accelerator plugin.