Originally macrocyclic compounds capable of assuming a basket (or 'calix') shaped conformation. They are formed from p-hydrocarbyl phenols and formaldehyde. The term now applies to a variety of derivatives by substitution of the hydrocarbon cyclo{oligo[(1,3-phenylene)methylene]}.
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Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1323 (https://doi.org/10.1351/pac199567081307)