The tricorder is one of the fundamental pieces of Star Trek Tech along with its other landing party constituents, the Phaser and the Communicator. The tricorder has appeared as an important story-telling device in every incarnation of Star Trek beginning in 1966 right through to the current movie version. But what was the nature of its first appearance? Was it used on some far-flung planet to detect some new type of life form? To find some exotic element on an alien asteroid? How about... as a tape recorder for a meeting?
In the early original series (TOS) episode "Mudd's Women", the scoundrel Harry Mudd is brought into the Enterprise's conference room for questioning. Sitting on the table throughout the meeting is a lone tricorder. No one ever acknowledges it, looks at it or touches it. It's just sitting. Is it on? Who knows? Basically, it's just so much set dressing. A background prop used as a futuristic tape recorder.
But fear not. In later episodes – and over the decades – the tricorder would establish its value by presenting key information to our characters in times of crisis. Time and again it would supply important, tale-turning data that let the writers tell their stories in a more concise way than would otherwise have been possible.
The tricorder would take on many forms, with many diverse capabilities. but it all started with this first innocuous glimpse in an early episode where it's basically just so much office equipment. But great things would come!
NOTE: while the tricorder was actually first shown on television in the episode "The Man Trap", "Mudd's Women" was filmed first and so was the first produced episode to show a tric.