"An active device or component is something which can provide an infinite energy or non-zero power for an infinite interval"
I learned this definition for an active device in a book, which is valid for capacitor, inductor and batteries (I have evaluated). I was wondering what is a "PN junction", "junctions of a transistor", "OPAMP", "JFET", "MOSFET"? I mean what are the above mentioned devices inside the quotation marks? Are they active devices or passive? If either one then please provide explanation for my understanding.
First, capacitors, resistors and inductors, all they are passive devices. An active device is one that is able to impose the voltage or the current in a given circuit.
That is, an active device (ideal) may be a voltage source which delivers a constant voltage, current whenever required. It can also be a source of current (ideal) that provides a permanent electric current to any power requirement.
OPAMP (operational amplifier), BJT, JFET (transistors), it is called active devices, because of its ability to manipulate and maintain voltage or constant current (within specified operating ranges, unlike the ideal elements).
Here, you can find a more detailed explanation.