C.W.
I have always been a parent who doesn't yell at my child and uses positive discipline and all that so I know when my daughter hit "terrible 2s" it was not because I'm an irresponsible parent. You WILL know when it happens. With my daughter it was literally like night and day. One day she was a great listener and was an "easy" child. She didn't throw many tantrums or anything. The next day she would not listen, started having time-outs, throwing tantrums in stores and at home, and would yell. It subsides as you guide them and teach them, but you WILL know when it happens lol. The day it happens you will be like oh she's just grumpy today and then it will happen the next day and the next and the next lol. It just means their attitudes and listening skills drastically change in a very short period of time... my daughter hit hers around 27 months old. They are testing boundaries and pushing that line
I totally agree with Hazel. While I am giving an example of the behavior as I have seen the drastic change people consider terrible twos, I don't like the phrase and don't think only twos are "terrible", it's just different than the compliant baby and takes a patience adjustment with parent/s.