What term to use, when the theoretical solution is absolutely inapplicable in practice due to other outstanding factors?
Let me give an example from my answer to Is there a maximum Isp for “exothermic chemical reaction rockets”?.
The most energetic reaction seems to be (although the claim is unsourced) oxidation of beryllium At 23.9MJ/kg it would purely theoretically allow 705 seconds of specific impulse. Purely theoretically, because beryllium oxide is a powder, so there's no adiabatic expansion of gas which creates propulsion.
And the comment by Uhoh:
I wish we had a better word than "theoretically" here. I can't think of one, but it's really just an absolute upper limit. Well know, unavoidable thermodynamic realities will probably lead to a lower number without question, so there's no actual "theory" that says it can be 705. But ya people frequently use "theoretically" like this.
I agree with the comment; the qualifier "purely theoretically" is clumsy here. The theoretical rule (all chemical energy converted to kinetic) simply doesn't apply in this case (the mechanism of conversion of chemical to kinetic energy doesn't work if the product of reaction is not a gas). Is there a better way to formulate the qualifier that 'this will not work in practice due to unrelated reasons'?