Opacity or opaque may refer to:

• Impediments to (especially, visible) light:
  • Opacities, absorption coefficients
  • Opacity (optics), property or degree of blocking the transmission of light
• Metaphors derived from literal optics:
  • In linguistics:
    • Opaque context, a term to describe the linguistic context of co-referential terms
    • Phonological opacity, a term in phonology
    • Semantic opacity, the opposite of semantic transparency
  • Opaque travel inventory, the market of selling unsold travel inventory at a discounted price
• Musical works:
  • Opacities (EP)
• Computer science:
  • Measure of obscuration of background by "painting"^{[clarification needed]} an image, e.g. alpha channel
  • Use of an opaque data type
  • Property attributed to data that can be interpreted only by using an external entity, e.g. magic cookie technology
  • Opaque binary blob, in network-engineering and computer-science technology, a large set of binary data (a blob) that looks like garbage to those unacquainted with its internal structure.