is a Latin
term used in biology
to identify a taxonomic
classification that has not been placed into the currently-understood taxonomic tree
. It literally means 'of uncertain placement' -- we have this organism, we've named it, but we don't know where it fits into the tree of life.
While this can happen with living animals (and it does), it is by far the most common when dealing with fossils. There are two reasons for this; the obvious one is that fossils give us less information, and often incomplete information, about plants and animals. Less obviously, we have other ways of dealing with living animals that avoids the use of the term Incertae sedis; for example, most snakes are lumped into the family Colubridae - not because they have characteristics that place them in Colubridae, but because that's where we put snakes that don't obviously go elsewhere.
In this sense, Colubridae is an collection of Incertae sedis, although it is not generally referred to as such. If we were to admit to this, we might use the term incertae familiae, specifying 'of uncertain family'. There are terms for other levels of uncertainty: incerti subordinis refers to an uncertain suborder, and incerti ordinis to an uncertain order.
By the rules of the International Code of Botanical Nomenclature Incertae sedis does not apply to the levels of genus and below. Uncertainty at these levels is indicated by a extensive set of abbreviations and markups: single quotation marks around a genus name indicates that that name is uncertain ('Agenus' aspecies); cf. proceeding a species name indicates that the classification is uncertain, and that it may actually be a member of a known species (Agenus cf. aspecies); aff. or sometimes n. sp., aff. or sp. nov., aff. indicates that a species is new and that the author doesn't want to firmly commit to a specific species name at this point, but it looks something like this other species (Agenus aff. aspecies); sp. indicates that no one has yet tried to assign a species, and ssp. means the same for a subspecies (Agenus sp. ssp.); a question mark may also be used to indicate that any given level is uncertain (Agenus? aspecies).
Incertae sedis is closely related to nomen dubium, and if you are not careful enough, may lead to a lapsus calami.