Originally: a member of the class Heterokontae (now usually called Xanthophyceae), containing yellow-green algae whose motile cells or zoospores have two dissimilar flagella (now rare). In later use: a member of the (proposed) phylum Heterokonta, a much broader group of single-celled and multicellular eukaryotes also including brown algae, diatoms, oomycetes, and other algal or fungus-like organisms, whose motile cells or zoospores typically have two dissimilar flagella, one with lateral filaments and the other smooth and shorter (or sometimes absent).

Of or relating to heterokonts.