A ping is a method to measure round trip time.
The process of "pinging" is to send a "ping". A "ping" is a network message which serves no other purpose than to get an immediate response from the receiver. This can be used to measure the round trip time. It can also be used to verify that another system is able and willing to respond to network messages at all. For example, some games use regular "keep-alive" pings so they notice when a player disconnects without logging out properly.
You can also measure round trip time by measuring responses to network messages which are not just pings but actually do have other practical purposes. You can use any network message which results in an immediate response from the other side and where the response can be clearly attributed to the message. But such a measurement might not be as accurate as with a dedicated ping message, because the receiver might require additional time to process and respond to the primary purpose of the message.
However, in general language usage, "ping" is sometimes used as a synonym for round trip time. Especially among players.