In dreams, it will depend on whether the helmet is being worn by someone else or by the dreamer.
If the former, it may have the same symbolism as the mask in that it prevents the wearer being seen.
If the latter, then it is a symbol of protection and preservation.
Lookup dream dictionary, dream symbols, dream meanings. The Online Guide To Dream Interpretation interpret your dreams, Helmet dream dictionary analysis, Helmet symbols, Helmet meanings, dream encyclopedia, islamic dream interpretation, dream views Helmet spiritual meanings of dreams.