(August 8, 2017)
The TARDIS is The Doctor's home and spaceship. TARDIS's were invented by the Time Lords, and contain a range of technology gizmos to manipulate time and space and energy systems and who knows what else. The TARDIS is therefore a central plot device in Doctor Who, and you could argue the TARDIS is also a character in the story since it has been demonstrated to have its own consciousness and will.
The word, TARDIS, has been treated as an "acronym" as if the letters have a meaning, just like the word RADAR is actually an acronym meaning either "RAdio Detection And Ranging" or "RAdio Direction And Ranging". The problem with thinking TARDIS is an acronym is language differences. Gallifreyans do not speak English, and therefore would not use an English-centric acronym as the meaning of the word TARDIS.
But, within the context of Doctor Who, several acronym/meanings have been given for TARDIS. Let's just go with that and hum a chant about timey-wimeyness.