Significance of the word iron in option strategies

I don't know how definitive it is, but Wikipedia has an answer:

The word iron in the name of this position indicates that, like an iron butterfly, this position is constructed using both calls and puts, by combining a bull put spread with a bear call spread. The combination of these two credit spreads makes the long iron condor (and the long iron butterfly) a credit spread, despite the fact that it is "long." This distinguishes the position from a plain Condor position (and the plain Butterfly), which would be constructed with all calls or all puts, by combining either a bull call spread with a bear call spread or a bull put spread with a bear put spread.

Investopedia (even less definitive) has a similar explanation:

The iron condor has a similar payoff as a regular condor spread, but uses both calls and puts instead of only calls or only puts.

So it seems that an "iron" spread is created with a combination of puts and calls, versus created with just puts or just calls.