Short answers, pulled from the story.
Masatoshi Nagatomi of Harvard University coined the phrase Nikaya Buddhism in the late twentieth century. He proposed this term as a non-derogatory substitute for Hinayana, which means early Buddhist schools.
The Pali word nikaya translates directly to group or assemblage. It refers to the collection of early Buddhist schools or non-Mahayana schools.
Early Buddhism in India is generally divided into various monastic fraternities known as nikayas that conventionally numbered eighteen though the actual count varied over time. The doctrinal orientation of each school differed somewhat, as did the number of piakas in their canon.
The Mahasamghika nikayas generally advocated the transcendental and supramundane nature of the buddhas and bodhisattvas while emphasizing the fallibility of arhats. They also emphasized the bodhisattva ideal and buddhahood was advocated over the ideal of becoming an arhat.
The Theravada nikaya only uses the Pali Canon which has three pitakas and does not contain separate literature for bodhisattvas. Walpola Rahula writes that at the end of a religious ceremony or an act of piety the bhikkhu who gives benedictions usually admonishes the congregation to make a resolution to attain Nirvana by realizing one of the three Bodhi - Sravakabodhi Pratyekabodhi or Samyaksambodhi - as they wish according to their capacity.