1. without advance planning

- they met accidentally

Similar word(s): circumstantially, unexpectedly

2. of a minor or subordinate nature

Similar word(s): incidentally

3. without intention; in an unintentional manner

Similar word(s): unintentionally

Sentences with accidentally as an adverb:

- He discovered penicillin largely accidentally.

- He accidentally exposed the bacteria to mold spores.