Is "X" kufr (reference question)

Kufr is often defined as disbelief; to what extent does this simple definition cover the meaning of Kufr in Islam? **Motivation:** We have a load of questions (see the hopefully growing list at the end) on the site where someone is in doubt ofhaving committed an act which can be regarded as kufr and asks whether Allah will forgive him or her. I think that all of these questions essentially have the same answer and it is counterproductive to repeat it all the time. The act of kufr itself and the backstory of the one asking the question is largely irrelevant. I'd like to cover as many aspects of Kufr as possible starting with a useful definition. And to as completely as possible, cover the topic of Kufr, in order to use this as a reference question. So please address in your answer the following question parts: - What is the definition of the term Kufr? - What constitutes Kufr (which acts and preconditions are necessary)? - Can Kufr be forgiven and is the process of repentance different than that of repenting from a sin? - What are the conditions for it (possibly) being forgiven? - What if such an act was made unintentionally or without knowing it was kufr? As the question is intended as a reference question, the answer should include evidence and have at least canonical level. Some examples of former questions: - https://islam.stackexchange.com/questions/69730/did-i-commit-kufr - https://islam.stackexchange.com/questions/68299/doing-black-magic-but-you-did-not-know-shirk-was-a-sin (feel free to expand the list even with posts including answers)