Созвучие > consonance is easy, but to get the first part, you have to look up musicological translations of the work of Taneyev's student Boleslav Yavorsky, who developed a double-mode theory. Evidently his устой/неустой has been translated into English as stability/instability. See here, for example:

"A general concept of instability vs. stability is the key to understand Yavorsky's system." (Granted, "understand" instead of "understanding" marks this as a non-native translation, but it's the best I could find.)
http://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=123...