Barrier

A barrier is an invisible block which appears transparent, but has mixed properties. A similar block is found in the Pocket and Pi editions, the invisible bedrock.

Obtaining
Barriers cannot be broken in survival mode.

Barriers are not listed in the creative inventory (similarly to command block) and can only be obtained using commands, such as.

Invisible bedrock cannot be selected or broken using any tools, and is unobtainable without the use of external programs. When held in the inventory, it uses the stone texture.

Natural generation
Invisible bedrock comprises the border wall surrounding old worlds. The wall does not generate past the height limit, allowing the player to walk atop it.

Usage
The block in the player's inventory shows up as a red box with a slash through it, but when placed is completely invisible.

Barriers are transparent to light, but interact with blocks and mobs like a solid block. All dependent blocks can be placed on a barrier, such as trapdoors or redstone, and it will suffocate mobs. Mobs cannot spawn on barriers.

In creative mode, if a player is holding a barrier block in their hand, all placed barrier blocks display the barrier icon as a particle. This effect is client-side, and if a player holds a barrier block in survival, the particle won't show up. Barrier blocks will not be shown if the particle setting is set to "minimal". Barriers cannot be pushed by pistons.

Invisible bedrock is transparent in rendering only, interacting with players and mobs as a solid block and blocking light. Any adjacent blocks are not rendered when viewed through invisible bedrock.

Trivia

 * The "No" symbol used is actually an inverted version of the ISO standard.