Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
arachnoid is a noun and adjective meaning "the middle of the 3 meninges". It carries 3 recorded senses.
the middle of the 3 meninges
air-breathing arthropods characterized by simple eyes and four pairs of legs
relating to or resembling a member of the class Arachnida
zoology
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for arachnoid come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.