Two planets form a semi-square when they are spaced about 45° apart, or half a square. It is a small tension, an irritation or a discreet friction rather than a clear-cut obstacle — often felt as a background irritation more than a definite blockage.
Angle: 45°. Orb used on this site: 2°.
Nature: classified as tense, in the same geometric family as the square (90°) and the sesquiquadrate (135°) — all multiples of 45°. See the Square entry for the major aspect of which this one is the softened version.