"Nakama" (仲間) is a common Japanese word, loosely defined as "people who are close to each other and who do something together".
In English, the word approximately translates to 'comrade', 'companion' or 'one of us (/them)'.
In the context of One Piece in particular, it is often used to denote a 'crewmate' or 'ally'.
In most dictionaries (広辞苑、大辞林、デジタル大辞泉)the definition of 'nakama' indicates a casual relationship.
On the other hand, the usage in One Piece corresponds most closely to the definition given in the Iwanami Japanese Language Dictionary (岩波国語辞典), where it has been defined as "心を合わせて何かをいっしょにするという間柄をかなりの期間にわたって保っている人。そういう間柄。", or "People who share the same feelings (lit. "whose hearts are united") who do something together, and who have maintained this relationship for a relatively long time."