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Does "Bitcoin" need to be capitalized? In formal writing or informal, what is the current usage?

Not, according to the note under Wikipedia article on 'Bitcoin.'

There is no uniform convention for bitcoin capitalization. Some sources use Bitcoin, capitalized, to refer to the technology and network and bitcoin, lowercase, to refer to the unit of account.[7] The WSJ[8] and The Chronicle of Higher Education[9] advocate use of lowercase bitcoin in all cases, however. [emphasis added]

Accordingly, Wikipedia by its own statement, "… follows the latter convention," that is, the "use of lowercase bitcoin in all cases."

[EDIT]
Clarification: The question being 'Does bitcoin need to be capitalized," the answer "Not" means that it is not necessary; it does not mean it is incorrect to do so. HTH.

According to en.bitcoin.it both forms are acceptable depending on context:

Capitalization / Nomenclature

• Since Bitcoin is both a currency and a protocol, capitalization can be confusing. Accepted practice is to use Bitcoin (singular with an upper case letter B) to label the protocol, software, and community, while




• bitcoins (with a lower case b) to label units of the currency.

Anonymity:

• When it comes to the Bitcoin network itself, there are no "accounts" to set up, and no e-mail addresses, user-names or passwords are required to hold or spend bitcoins.

• Concept and network, capitalised: Bitcoin as in the Bitcoin Foundation or Should the banks allow Bitcoin to ...




• Currency, lower case: bitcoin as in I paid a bitcoin for this




• Currency symbol, all capitals: BTC as in Price: 1BTC

Bitcoin.org uses capitalised Bitcoin to refer to the concept and lowercase for the actual currency.

Wikipedia

Production 25 bitcoins per block (approximately every ten minutes) until mid 2016.

Subunit

 10−8 satoshi[2]

 10−6 bit or μBTC

 10−3 mBTC

Symbol BTC, XBT

There is no uniform convention for bitcoin capitalization. Some sources use Bitcoin, capitalized, to refer to the technology and network and bitcoin, lowercase, to refer to the unit of account. The WSJ and The Chronicle of Higher Education advocate use of lowercase bitcoin in all cases, however. This article follows the latter convention.

A couple of years ago, the technology magazine where I worked wrote a lengthy feature article on the Bitcoin phenomenon, and we took great pains in the story to distinguish between Bitcoin as a currency or concept on the one hand and individual (imaginary) bitcoins—that is, units of currency in the Bitcoin system—on the other.

Unfortunately, many occurrences of the word were ambiguous, so we had to make a series of close calls about whether to put a particular instance of the word in the initial-cap pile or in the lowercase pile. When we published the story, several readers wrote in to complain that we evidently had no idea whether to capitalize the word or not, since it appeared sometimes capped and sometimes not.

So we went back to our in-house style guide and removed the elaborate explanation for when to capitalize Bitcoin and when to lowercase it, and replaced it with the simple (and arbitrary) rule to lowercase bitcoin in all occurrences. The result: Readers stopped complaining about our inconsistency; and (not insignificantly) we never heard a peep from legal representatives of the Bitcoin enterprise.