Learner's definition of ONTO
1 : to a position that is on (something, such as a surface, area, or object)
— often used figuratively
We climbed onto the building's roof.
The book fell onto the floor.
The water spilled onto the floor.
The cowboy leaped onto his horse.
Transfer the data onto a disk.
Turn left onto Third Street at the traffic light.
2 : in a direction that allows you to get to or see (something)
The door opens onto a balcony. [=when you go through the door you are on a balcony]
The dining hall opens onto a courtyard. [=you can enter the courtyard from the dining hall]
The bedroom window looks (out) onto the bay.
3 a — used to say that someone knows about what someone is doing or has done b — used to say that someone is becoming aware of or is finding something
Scientists believe they are onto something big. [=scientists believe they are close to making an important discovery]
When the crowd responded to the show so positively, we realized we were onto something. [=we had done/discovered something important, special, etc.]
4 chiefly British : in the process of talking to (someone) usually to tell or ask something