Incertae sedis

In taxonomy, the term incertae sedis (abbreviated inc. sed.) is used for a taxon when its broader relationship to other taxa is unknown. The first person who used the term in botany was probably Antoine-Laurent de Jussieu. In his book "Genera Plantarum" of 1789 he listed a few "plantae incertae sedis". 
Reasons why a taxon might be classified as "incertae sedis" include: