- (idiomatic, reciprocal pronoun) Used of a reciprocal relationship among a group of two or more people or things; compare each other.
- The raw recruits helped one another get over the first few days.
- Rainy days seemed to follow one another all summer.
idiomatic, reciprocal pronoun
Usage notes 
Some usage guides proscribe "each other" for two entities and "one another" for more than two; this distinction is not observed in practice. The Oxford English Dictionary describes the pronoun as referring to ‘two or more’; Fowler's suggests that the distinction ‘is neither of present utility nor based on historical usage’. The Merriam-Webster Dictionary of English Usage notes that ‘A few commentators believe the rule to be followed in “formal discourse”. This belief will not bear examination: Samuel Johnson's discourse is perhaps the most formal that exists in English literature, and he has been cited in violation of the rule.’
See also