Also contains definitions of: free spiro union, spiro atom
https://doi.org/10.1351/goldbook.S05884
A union formed by a single atom which is the only common member of two rings. The common atom is designated as the spiro atom. A free spiro union is one which constitutes the only union direct or indirect between the two rings.
Source:
Blue Book, p. 37 [Terms] [Book]