adverb
Etymology: Middle English at on, from at + on one
1.
a. : in a state of unity of feeling : in harmony : in a closely united friendly relationship
the poet felt at one with nature
b. archaic : into peaceful agreement : into harmony — used especially with bring, make, or set
he showed himself unto them as they strove, and would have set them at one again, saying, Sirs, ye are brethren — Acts 7:26 (Authorized Version)
2. : of an identical or sympathetic frame of mind : of the same opinion : in a state of agreement
on these points we are at one
3. obsolete : together