"Запаздывание" or "опоздание"

Question

What's the difference between those two words? In the dictionary, it is translated as delay for both.

Answer 1

"Запаздывание" is mostly a technical term. For example, a delay between input and output signals is "запаздывание сигнала", and you can't say "опоздание сигнала". Other examples:

Запаздывание потенциала - retarded potential
Система с запаздыванием - system with delay
Запаздывание затвора (фотоаппарата) - Shutter lag

"Опоздание" is a colloquial word for everyday use. For example:

Извините за опоздание - Sorry, I'm late

Answer 2

A very short answer is

запаздывание = being about to get late, lagging behind schedule (often chronic)
опоздание = being late (after the fact), having been late (usually a one-off occurrence)

Generally translation into English is heavily dependent on the context.