Learner's definition of ONTO
to a position that is on (something, such as a surface, area, or object)
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.
— often used figuratively
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.
— used to say that someone knows about what someone is doing or has done
— 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.]
in the process of talking to (someone) usually to tell or ask something