We don't have synonyms for mirrorless yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are errorless, morrowless and marrowless.
(n) :
(photography) A mirrorless interchangeable-lens camera, similar to a digital SLR, but usually made smaller by incorporating an electronic viewfinder instead of a mirror and prism.
(n) :
(photography) A mirrorless interchangeable-lens camera, similar to a digital SLR, but usually made smaller by incorporating an electronic viewfinder instead of a mirror and prism.