Definitions of barrier:

• noun:   a structure or object that impedes free movement
• noun:   any condition that makes it difficult to make progress or to achieve an objective

  Example: "Intolerance is a barrier to understanding"

• noun:   anything serving to maintain separation by obstructing vision or access
• name:  A surname (rare: 1 in 50000 families; popularity rank in the U.S.: #6936)