1. Of or relating to the largest order (Passeriformes) of birds which includes over half of all living birds and consists chiefly of altricial songbirds of perching habits. (merriam-webster.com)
2. Perching birds mostly small and living near the ground with feet having 4 toes arranged to allow for gripping the perch; most are songbirds; hatchlings are helpless (wordnetweb.princeton.edu)
3. A bird of the order Passeriformes, which includes more than half of all bird species. Sometimes known as perching birds or, less accurately, as songbirds, the passerines form one of the most diverse terrestrial vertebrate orders; with over 5,000 identified species, it has roughly twice as many species as the largest of the mammal orders, the Rodentia. It contains more than 110 families, the second most of any order of vertebrates (after the Perciformes) (wikipedia.org)
4. Any bird of the order Passeriformes, which comprises more than half of all bird species. (wiktionary.org)
Syn: passeriform bird, perching bird
Word origin: Derived from Passer domesticus, the scientific name of the eponymous species (the House Sparrow) and ultimately from the Latin term passer for Passer sparrows and similar small birds.