We don't have synonyms for first-order yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are first order and first-aider.
(adj) :
(mathematics, logic) Of one of a series of models, languages, relationship, forms of logical discourse, etc., being the simplest one or the first in a sequence.
(adj) :
(mathematics, logic) Of one of a series of models, languages, relationship, forms of logical discourse, etc., being the simplest one or the first in a sequence.