From mesos (middle) and ouranos (heaven). The highest point of the sky — the zenith or mid-heaven where the sun stands at noon, visible to all. Used in Revelation for angels making proclamations from the highest, most visible point of the sky.
In Revelation, an angel flies in midheaven proclaiming the eternal gospel to every nation (Revelation 14:6). Another angel in midheaven announces God's judgment (Revelation 8:13). The use of this word emphasizes that God's final messages are inescapable and universal — broadcast from the highest point, seen and heard by all humanity. No one will be able to claim ignorance.