Incertae sedis
Incertae sedis is a taxonomic designation for an organism or group whose position within a classification is uncertain or unresolved.
Phylogenetic inference: The use of evidence to reconstruct the evolutionary relationships among organisms. Phylogenetic analyses can test where an uncertain taxon belongs.
Biological classification: The systematic arrangement of organisms into groups based on shared characteristics and relationships. Incertae sedis marks uncertainty within a biological classification.
Nomen dubium: A scientific name whose application is uncertain because its type material is inadequate or ambiguous. It concerns uncertainty about a name's identity, not a taxon's placement.
Protist: A eukaryotic organism that is not classified as an animal, plant, or fungus. Many protist lineages have historically had uncertain positions in broader classifications.
Cladistics: A method of classification that groups organisms by shared derived characteristics and common ancestry. Cladistic grouping makes unresolved branches visible when evidence cannot place a taxon confidently.
Taxonomy: The science of naming, describing, and classifying organisms. The designation is used in taxonomic practice.
Nomen nudum: A proposed scientific name that was not published with the description required for formal availability. It describes a nomenclatural defect rather than uncertain classification.
Fossil: Preserved remains, traces, or chemical evidence of organisms from the geological past. Fragmentary fossils can be difficult to place within established groups.
Molecular phylogenetics: The study of evolutionary relationships using DNA, RNA, or protein sequence data. Sequence comparisons can clarify placements that morphology leaves uncertain.
Taxonomic rank: A level in a biological classification, such as species, genus, family, or order. Uncertainty can concern a taxon's position at a particular rank.