noun (n)

• rounded brimless cap fitting the crown of the head(noun.artifact)

  source: wordnet30
• a herbaceous plant of the genus Scutellaria which has a calyx that, when inverted, resembles a helmet with its visor raised(noun.plant)

  Synonym:
  helmetflower
  

  source: wordnet30
• the dome of the skull(noun.body)

  Synonym:
  calvaria
  

  source: wordnet30
• A cap which fits the head closely; also, formerly, a headpiece of iron sewed inside of a cap for protection.(noun)

  source: webster1913