In general, a horizon is a boundary line between an area that you can
see and an area that you cannot see. In astronomy, there are at least
four relevant meanings of horizon, of which three are closely
related. The horizon is:
The line along the sky that is exactly midway between (at 90
degrees from) the zenith and the nadir, at a height of 0 degrees.
The imaginary boundary line in the sky between the land (or sea)
and the air, if the Earth were perfectly round (smooth). If your eye
were halfway in the ground (or sea), then this horizon would be the
same as the one of explanation 1 (though the difference is only
important if you are kilometers or miles above the land or sea).
The boundary line in the sky between the air and the things on the
ground (or sea). If mountains or buildings are nearby, then this
horizon is not a straight line. On a dead calm sea, this horizon is
the same as the one of explanation 2.
The horizon of a black hole is the edge of the region around a
black hole from where you cannot escape.
If an astronomical text mentions a horizon but doesn't explain which
one it is, then you can assume that it is the horizon of explanation