Brainwallet
A memorized Bitcoin seed phrase
Example
My brainwallet is foolproof. You see, the first letter of my seed phrase starts with w, and George W. Bush was the 43rd U.S. president. 43 is also the number of Richard Petty's racecar, which he used to win the Winston Cup Championship. So, the first word of my seed phrase is obviously winston
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Those who do not want to write down the seed phrase associated with their Bitcoin wallet may memorize the phrase instead. In this case, the person may refer to the phrase as their brainwallet - because the ability to access their wallet relies on their ability to remember their seed phrase.
What is a seed phrase?
A seed phrase is a list of words a Bitcoin owner can use to access their Bitcoin. Seed phrases are generated by and linked to Bitcoin wallet software, which Bitcoin owners use to store their Bitcoin. Using a seed phrase, Bitcoin owners can access their Bitcoin from any computer that has their particular wallet software installed. This allows Bitcoin owners to "back up" their Bitcoin.
In most cases, Bitcoin owners write down their seed phrases, so they do not forget them and lose access to their Bitcoin. However, some paranoid and/or overconfident Bitcoin owners may attempt to commit their seed phrases to memory, using mnemonic devices or repetition. These Bitcoin owners have trusted their Bitcoin's continued existence to their brains - turning their brains into brainwallets.