apical (basal, equatorial)

https://doi.org/10.1351/goldbook.A00415
In trigonal bipyramidal structures (e.g. a five-coordinate trigonal bipyramid with phosphorus as
central atom
) the term apical refers to the two positions that are collinear with the
central atom
or to the bonds linking these positions to the
central atom
. The three equivalent bonds (or positions) in a plane passing through the
central atom
and perpendicular to the direction of the apical bonds are described as
equatorial
. (See
axial, equatorial
for alternative use.). The term apical is also used for the bond pointing from the atom at or near the centre of the base to the
apex
of a pyramidal structure. The positions at or near the base of the pyramid, or the bonds linking those positions to the
central atom
of the base are described as
basal
. The apical bonds have also been called axial.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2199 [Terms] [Paper]