TARDIS force field

The TARDIS force field was a force field surrounding the outer plasmic shell of the craft. It was designed to protect the occupant(s) both from harm (though it was far from invulnerable) and from the Time Vortex and other environments dangerous to its pilot.

See real world interface.

Dduring his fourth incarnation, the Doctor reconfigured the force field of his TARDIS, a Type 40, into a corridor through which he could safely traverse a vacuum. (DW: The Horns of Nimon). In his eleventh incarnation he repeated this operation to permit River Song to transfer from another ship in space. (DW: The Time of Angels).

After the Last Great Time War, the force field was non-default, and a tribophysical waveform macro-kinetic extrapolator was required for the TARDIS to work instead. (DW: The Parting of the Ways)