Currently, an E Ticket is a ticket that is ordered and paid for online. You don’t have the official "ticket" in your hand. It is an "E-lectronic ticket".
An E ticket used to be something else:
At Disneyland when I was a kid (I was born a couple of years before it was built) you got a book of tickets for the rides. The A tickets were for the baby rides like Dumbo that only went around and around while you sat in a fiberglass elephant.
There were also B, C and D tickets for gradually more fun rides. There was also a special ticket for "Great Moments with Mr. Lincoln" so you didn’t have to use one of your ride tickets to watch an audio-animatronic Abe Lincoln give a speech.
But, the best ticket of all was the E ticket. This was for the best rides at Disneyland. The Haunted Mansion, Pirates of the Caribbean, and Space Mountain were E ticket rides.
Later on, Disneyland switched to a one price for all rides admission fee, and the ticket books were no more. Only us old timers would remember what an E ticket was.
It became a figure of speech somewhere down the line. If you went someplace and had a great time, you would say that it was an E ticket ride.
Now, if I say E ticket, whippersnappers think I’m talking about something else.
Boo hoo. Time marches on, and I have lost one of my favorite figures of speech.