different dictionaries have different definitions
here is summary of the definition of parasite from various medical dictionaries and the "other species" distinction is missing in place of "other organism". Note one definition even offers the example of a co-joined twin as a parasite and so that kind of eliminates any doubt about whether something of the same species can also be a parasite
parasite /par·a·site/ (par´ah-sīt)
1. a plant or animal that lives upon or within another living organism at whose expense it obtains some advantage; see symbiosis.
2. the smaller, less complete member of asymmetrical conjoined twins, attached to and dependent upon the autosite.parasit´ic
malarial parasite Plasmodium.
obligatory parasite one that is entirely dependent on a host for its survival.
Dorland's Medical Dictionary for Health Consumers. © 2007 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
par·a·site (pr-st)
n.
1. An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host.
2. In conjoined twins, the usually incomplete twin that derives its support from the more nearly normal fetus.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
Parasite
An organism that lives in or with another organism, called the host, in parasitism, a type of association characterized by the parasite obtaining benefits from the host, such as food, and the host being injured as a result.
Mentioned in: Antihelminthic Drugs, Antimalarial Drugs, Antiprotozoal Drugs, Babesiosis, Brucellosis, Chagas' Disease, Cryptosporidiosis, Cyclosporiasis, Fluke Infections, Stool O & P Test, Vulvovaginitis
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.
parasite
[per′əsīt]
Etymology: Gk, parasitos, guest
1 an organism living in or on and obtaining nourishment from another organism. A facultative parasite may live on a host but is capable of living independently. An obligate parasite is one that depends entirely on its host for survival.
2 See parasitic fetus. parasitic, adj.
Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.
parasite [par´ah-sīt]
1. a plant or animal that lives upon or within another living organism at whose expense it obtains some advantage; see also symbiosis. Parasites include multicelled and single-celled animals, fungi, and bacteria, and some authorities also include viruses.Those that feed upon human hosts can cause diseases ranging from the mildly annoying to the severe or even fatal. (See accompanying table.)
Types of parasites.
2. parasitic fetus.
adj., adj parasit´ic.
accidental parasite one that parasitizes an organism other than the usual host.
facultative parasite one that may be parasitic upon another organism but can exist independently. incidental parasite accidental parasite.
malarial parasite Plasmodium. obligate parasite (obligatory parasite) one that is entirely dependent upon a host for its survival.
periodic parasite one that parasitizes a host for short periods.
temporary parasite one that lives free of its host during part of its life cycle.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
parasite,
n an organism living in or on and obtaining nourishment from another organism.
Mosby's Dental Dictionary, 2nd edition. © 2008 Elsevier, Inc. All rights reserved.
parasite
a plant or animal that lives upon or within another living organism at whose expense it obtains some advantage. See also symbiosis.
Among the many parasites in nature, some feed upon animal hosts, causing diseases ranging from the mildly annoying to the severe and often fatal. Parasites include multicelled and single-celled animals, fungi and bacteria. Viruses are sometimes considered to be parasites. However, the commonest use of the word refers to the multicellular helminth, arachnid, crustacean (copepod) and arthropod parasites.
accidental parasite
one that parasitizes an organism other than the usual host.
facultative parasite
one that may be parasitic upon another organism but can exist independently.
incidental parasite
accidental parasite.
obligate parasite, obligatory parasite
one that is entirely dependent upon a host for its survival.
periodic parasite
one that parasitizes a host for short periods.
temporary parasite
one that lives free of its host during part of its life cycle.
Saunders Comprehensive Veterinary Dictionary, 3 ed. © 2007 Elsevier, Inc. All rights reserved
