To me, it's all the same thing. When I drew the Reiki signs/symbols/foci/sigils/glyphs on someone I was working on, she asked me, are those sigils? I said yes. Because it is. It's focusing the Power of Creation itself. Invoking it, bringing it in, using it to ground, to heal, to align.
I often say symbols because that's how they were referred to when I was taught them. But you can call them anything you want. They are images of power, words of power, intention made manifest; head hand and heart, body mind and spirit.
Sigil has an occult feel, and there are many sigils out there that aren't Reiki specific. Not all sigils are beneficent. The intention of the creator of the sigils is very important to take into account.
Intention is key. If someone is using the intention to restore and heal, that sounds good, but what are they restoring and healing? If you restore and heal an invading bacteria you can cause the bacteria to flourish in the body they have invaded.
Rest assured, however, that the Reiki symbols are pure. You can call them foci/sigils/signs/glyphs, but most important, above all, they are pure. You can't hurt someone with the symbols unless you actually intend to bastardize them.
So far, my students are good people, not a-holes. So we're good. :)
Effectively the point of this is that you can use different language to talk about the same concept in order to reach across the aisle. You can even talk about the Reiki symbols as Godly symbols to help you do the healing work of the Lord for your Christian clients.
Meet people where they are at.
Thoughts? Questions? Leave them in the comments!