A human on earth can say "I define myself as citizenshipless" or "I define myself as a human without a citizenship" and alike ("I am global" or "I am free from citizenships"), and yet, that human could use a passport and/or an ID card in practice, so what is a citizenship exactly?
I assume that citizenship is any time period in which all authorized officers (or enough officers) of a given state would agree to issue citizenship documents (for example, a passport) for that human.
Citizenship is essentially an imaginary label that sovereign states assign to people to say "this person is one of us".
How those imaginary labels are granted, recorded, proved, maintained etc. is up to the sovereign state to decide.
I assume that citizenship is any time period in which all officers of a given state would agree to issue identification documents for that human.
Not accurate. Identification documents simply identify persons. They may or may not convey information about citizenship. Some (like driver's licences) do not. Other (like alien's passports) explicitly mean that the holder is not a citizen of the issuing sovereign state.
Conversely, having troubles to get a passport does not necessarily mean that the person is not a citizen. In some circumstances it may be difficult to prove citizenship (e.g. emigrated as an infant with no birth record), and court proceedings may be needed to convince the authorities.
I person can say "I define myself as citizenshipless" (more often called being stateless), but saying so does not make it legally effective, Most people on earth get citizenship somewhere at birth, either by place (ius soli) or by decent (ius sanguineus) Some get more than one citizenship, or can squire an additional one later.
Each country defines citizenship via its laws, regulations, and administrative actions. If a person P is a citizen according to the laws of country A, that person remains a citizen unless something happens that changes that status. That may be some thign that P does, or it may be something that country A does, oe both. But as long as A says that P is a citizen, A's laws will treat P as a citizen, including any obligations.
Some countries, such as the US, allow a citizen to make a formal declaration, in front of an official, that the citizen renounces citizenship, and this ends P's citizenship. Some countries will terminate P's citizenship if P applies for citizenship in country B, or join's B's military, or commits one of a list of crimes, or fights in a war against A. Some countries have at times passed laws declaring that specific people, or members of specific groups, are no longer citizens. And some countries may not have any way for P's citizenship to end.
In the end it is the laws and official declarations of the country that count. Citizenship is a legal status, like marriage or holding a particular license. Like those, it is the laws and any implementing regulation of the country A that define it in that country.
Note that A may consider P a citizen of A, but country B may not. Each country's laws will apply in that country.
Being stateless that is, not being a citizen of any country, usually has significant disadvantages, but some people have intentionally become stateless.
I assume that citizenship is any time period in which all officers (or enough officers) of a given state would agree to issue citizenship documents (for example, a passport) for that human.
Not correct. There are several reasons why a county might refuse to issue a passport or other document that certifies citizenship to a person who is nonetheless considered to be a citizen. Most countries will not issue a passport to a minor. Some countries will not issue a passport to a person who has been convicted of certain crimes. Many countries will not issue a passport to anyone awaiting trial on a serious crime. The US has, on a number of occasions, refused to issue passports to people whose political opinions it disapproved of, when it did not want them making speeches outside the US. Other countries might act similarly. As to other documents that indicate citizenship, some countries do not really have any, others do. But many have requirements beyond bare citizenship for issuance of such documents..
Citizenship in relationship to passports and other countries, is a claim made by a State saying that a person “belongs” to them. It is a statement of support and an implied threat.
Since it is a claim by the State, how the individual feels about it is to an extent irrelevant. Someone that wants to renounce US citizenship for example doesn’t get to just say “I define myself as stateless and not as a US citizen”, there’s a formal process to go through in which the US government agrees to renounce ownership of the individual. Until the US relinquishes it’s claim, it will still consider you under its jurisdiction and and other countries will in most cases acquiesce to that view.
For instance, if such a person was trying to get out of the fact that the US considers worldwide income when considering whether you have to file and pay taxes, publicly stating that (for instance taking out an ad in the local newspaper stating it), would not help them when the US come for it’s money and wants to put that person in jail for tax evasion.
Issuance of passports to individuals is done when the State believes that someone is a citizen and is qualified to receive a passport. This is almost always done at the request of the individual or guardian and not instigated by the country itself, and frequently includes the payments of fees as well as supplying documents that support the claim.
I do not know of any circumstances where a mere “I define myself as” would make any difference whatsoever.
It can be useful to have a look at one of the most unusual citizenships, namely EU citizenship. EU citizenship does not have identity documents. But what it does do, like all citizenships, is grant certain exclusive rights (as opposed to human rights, which would apply to everyone in the EU).
One of these EU rights is the right to travel in the EU. Non-citizens require visa for that. You can "define" yourself to not have EU citizenship; that just means that you voluntarily do not choose to exercise those rights.
The EU citizenship is also unusual in that it grants rights, but imposes no requirements. There is no EU tax on its citizens, nor is there an EU conscription. If you would try to deny citizenship of a country which does have conscription, their military will be less than impressed.
