adverb
mutually; with each other; reciprocally
Used when two or more parties do something to or with each other. Often interchangeable with お互いに, but 互いに is slightly more formal or written.
See also: お互いに
私たちは互いに助け合うべきだ。
We should help each other.
二人は互いに顔を見合わせた。
The two looked at each other.