Barrier

A barrier is an invisible block that appears transparent, but has mixed properties.

Obtaining
Barriers cannot be broken in vanilla survival mode.

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

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

Barriers are transparent to light and do not block a Beacon. Fences, Iron Bars, Glass Panes and similar blocks will not visually connect with barriers. Barriers interact with blocks and mobs as a solid block. All dependent blocks can be placed on a barrier, such as torches 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 display. Barrier blocks will not be shown if the particle setting is set to "minimal". Barriers cannot be pushed by pistons.

Trivia

 * The symbol used is not compliant with ISO 3864-1.
 * Invisible bedrock, a technical block in Bedrock Edition and New Nintendo 3DS Edition, shares many features with the barrier block.
 * The barrier block is in the Xbox 360 Edition of the game, but requires an inventory editor/external tool.