Who or what is the "other M" in Metroid: Other M?

Spoilers

Who or what is this "Other M" that is referenced in the game's title? Is there anywhere this is stated definitively (or pretty clearly implied) in the game? I've finished the whole game, including the return to the bottle ship to get Adam's helmet, and I don't remember anything. (I have not played on hard mode though.)

I originally thought it meant the other metroids that were being bio-engineered to be cold-resistant. But then I thought it must be referring to something other than metroids ("Metroid" is already in the title, so the other "M" must be something else), so now I'm going on the theory that MB is the other "M". But I'd love to know one way or the other if there is any clear answer I missed.

I've not seen anything in the game that explains what the title actually means.

I saw a debate about this on a gaming forum. The debate seemed to be between two theories:

Other Mother - Standing for MB.
Other Metroid - Standing for the Metroid Queen.

Like you, I assume it's the former.

As I understood it, it alludes to the plot twist late in the game when we learn "Madeline Bergman" is in fact "MB", and another woman altogether is the real Madeline Bergman. Both share initials, each beginning with M.

They could have called it Metroid Other MB, but that would have spoiled the plot.