1 the horizon : the line where the earth or sea seems to meet the sky
We sailed toward the horizon.
The sun rose slowly over/above the eastern horizon.
2 a [count] : the limit or range of a person's knowledge, understanding, or experience
Reading broadens/expands our horizons.
2 b [count] : the limit of what is possible in a particular field or activity
These discoveries have opened up new horizons in the field of cancer research.