The RO stands for reverse osmosis. That took mee a minute to figure out until one day I was staring at my filter and that lightbult started to sparkle a bit. Another term not on the list (I didn't read all the posts, sorry if someone got this already) is chlorosis or the yellowing of the leaves. Another term with that one is interveinal chlorosis which is the same, but, you guessed it, between the veins.