Chromera velia, also known as a "chromerid", is a unicellular photosynthetic organism in the superphylum Alveolata. It is of interest in the study of apicomplexan parasites, specifically their evolution and accordingly, their unique vulnerabilities to drugs.

The discovery of C. velia has sparked renewed interest in protist research, concerning both algae and parasites, as well as free-living unicells. Strict separation of botanical protists (algae) and zoological protists (protozoa) has been conventional but C. velia may be regarded as a good example of a bridge linking both categories.

C. velia has typical features of alveolates, being phylogenetically related to Apicomplexa (a subgroup of alveolates), and contains a photosynthetic plastid (chloroplast) while the apicomplexans have a non-photosynthetic plastid called the apicoplast. C. velia is also related to another subgroup of alveolates, the dinoflagellates of which most are photosynthetic.

C. velia uses metabolites (reduced carbon) from its plastid as its primary energy source. The same is true of the algal cousin of C. velia, another chromerid Vitrella brassicaformis. Together these are phylogenetically the closest known autotrophic organisms to apicomplexans.

Parasites in the apicomplexan genus Plasmodium are the causative agents of malaria. Studies of C. velia and V. brassicaformis are broadly useful for understanding the biochemistry, physiology and evolution of the malaria parasite, other apicomplexan parasites, and dinoflagellates.

"Apicoplast" is a specialised word, derived from the word "plastid". Initially the word plastid was more suitable than "chloroplast" when describing organelles of apparent algal descent in any protist, but that lack any chlorophyll or light absorbing pigment. Those found in apicomplexan parasites are a prominent example. The majority of members of the apicomplexan lineage still contain a genome in the plastid, indicating the organelle of the lineage's ancestors was once photosynthetic, but these plastids have no light absorbing pigments or light reaction machinery.

While Chromera velia contains a photosynthetic plastid, the majority of apicomplexan relatives contain a non-photosynthetic plastid, and the remainder contain no plastid. The ancestral photosynthetic plastid of ancestral apicomplexans may have been very similar to the plastid of C. velia or the plastid of V. brassicaformis.

Just as the term "plastid" has become widely adopted for chloroplast-derived organelles of non-photosynthetic protists, the term "apicoplast" has also gained acceptance for the plastid of apicomplexans. In current usage, the term plastid may even be used to describe the chloroplast of any photosynthetic organism, and so has a general non-discriminatory use.