Heterotrophic nutritional mode of some photosynthetic organisms in which ingestion of food particles is required for existence. 

Definition by Selosse et al. 2017, Ecology Letters, 20: 246–263.
We define mixotrophy as the physiological feature of an organism whose cells (at least some cells, for multicellular organisms) use both photosynthesis and external organic matter as a source of carbon and/or non-carbon elements. Thus, we do not consider as mixotrophic an autotrophic organism with heterotrophic cells using organic matter of internal origin. Our definition does include endosymbiotic consortia where photosynthetic cells are embedded in an otherwise heterotrophic cell, which also uses external organic matter. Three main mechanisms allow mixotrophy under our definition: Absorbotrophic (=osmotrophic) mixotrophy, Biotrophic (= symbiotic) mixotrophy, and Necrotrophic (= predatory) mixotrophy.