Depending on where I read, some seem to say that pillar 3b is always a life insurance; or any insurance like unemployment insurance; or no insurance at all and all savings that you allocate to your retirement are considered 3b.
I couldn’t find any official definition anywhere,
Anybody knows?
3b is nothing. It’s just an advertising term that makes it seem official by copying the pillar naming scheme. You buying ETFs or just having money in your bank account is also “3b” 
Seeing things like this makes me angry though:
Die Beiträge können allerdings nicht von den Steuern abgezogen werden. Das Guthaben muss als Vermögen versteuert werden. Die Auszahlung hingegen ist steuerfrei und der Zeitpunkt ist frei wählbar.
They are trying to invent advantages where there are none. Of couse you can use your saved money whenever you want without having to pay taxes. 
(And of course you have to pay wealth taxes on your savings instead of deducting them…)
Isn’t there an actual 3b with tax advantage in a few cantons in Romandie?
Geneva and Fribourg/Freiburg allow fiscal deductions on the cantonal level. They’re not named 3b in the law but seem to fit roughly life-insurance premiums and the interests of savings (but not the savings themselves).
So, 3b would be any kind of savings/life-insurance we do outside of 3a, of which only life-insurance premiums and interests accrued are deductible, only in a few Kantons.
For more details (but I’m in no mean a specialist, just your average guy looking for the relevant texts of law on the cantonal websites):
For Fribourg, it’s in the:
Loi sur les impôts cantonaux directs / Gesetz über die direkten Kantonssteuern: https://bdlf.fr.ch/app/fr/texts_of_law/631.1/versions/6673?all_languages=true&diff=split
Art. 34, al. g)
It allows a deduction for life-insurance premiums, health-insurance premiums and accident-insurance premiums as well as interests accrued on savings (but not the savings themselves).
My understanding is that health- and accident- insurance premiums are handled separately, as most Kantons do. The part that could be considered 3b (but isn’t named in that way in the text of law) are life-insurance premiums (750.- for single taxpayers / 1’500.- for married couples) and interests of savings (150.- for singles / 300.- for married couples).
I can’t wouch for Geneva because both their website and Law structure looks awfuly convoluted and not browsing friendly but it’s apparently handled by the Loi sur l’imposition des personnes physiques [FR]: https://www.ge.ch/legislation/rsg/f/rsg_d3_08.html
Art. 31, al. d)
There too, it seems to apply to life-insurance premiums and interests accrued on savings. The amounts allowed seem, there again, awfuly convoluted but as I understand it:
2’200.- for singles +900.- per dependant (+1’800.- if not subject to BVG / LPP)
3’300.- for married couples +900.- per dependant (+1’800.- if neither partner is subject to BVG / LPP, +1’350.- if only one of them is).
There again, the term “3b” doesn’t appear in the law.
Other rules may apply for businesses. I’m not specialized in the laws of either Kanton so may completely be wrong in my interpretations (study it for yourself / hire an advisor if you want to be sure to take advantage of it correctly).
The term pillar 3b denotes any private, non-tax-privileged providence. In addition to permanent life insurance, the term is also used for term life insurance and disability insurance. Basically any insurance which can be offered under the pillar 3a. In these cases the term pillar 3b is used to denote the regular (non-restricted and non-tax-privileged) model.
It’s more of a colloquial term (like pillar 2b for voluntary pillar 2 benefits) used by companies, and is not widely used in Swiss laws or legal documents.