The word Siphon changed its meaning in Oxford Dictionary after 100 years!!!It was given as 'an instrument working by atmospheric pressure'.But Stephen Hugus of Queens Land Technical University Australia suggested that the meaning is 'an instrument working by gravity' and was accepted.
For classical stadium, i.e. a semi-circular running track then the plural is stadia. If referring to a modern-day multi-event stadium, then the plural is stadiums.