First, what is a Pitch?
"Pitch" in the business context was a term that was coined to explain the art of describing a product, a company, or an idea to an audience, thereby tempting them to buy it.When you are pitching your startup, you are giving away information about your company and its projected success to convince a potential investor to purchase shares in your company and it's future success (equity).
And, what is a Deck?
Way before powerpoints, keynotes, and even computers came to light, people used to gather in meeting rooms with dim lights and overhead projectors to explain ideas and concepts. They used viewfoils that had diagrams and charts drawn on it (which were loaded on to the overhead projector) as an aid to their speech. These individual foils were called slides, and when a set of foils were put together for a presentation, it was called a deck of slides.
These terms stuck over time, and now you will often hear presentations, slides, and slide deck all used interchangeably.A "deck," therefore, is a collection of visually and textually rich, meaningful slides that acts asa visual aid to a speech. These sets of slides are usually in the form of static Powerpoint, keynote, Google slide format, or in other better engaging.