Sometimes you may see the word “C2DM" in old Android legacy code. What is that? What it means?

C2DM means Cloud to Device Messaging.

It was used in old times to enable developers to send data from servers to Android applications (push notifications).

C2DM was originally launched in 2010 and it was available starting from Android version 2.2.

In 2012, Google deprecated C2DM and replaced it with Google Cloud Messaging Service (GCM) and then shut down C2DM completely in 2015.

Then in April 2018, Google deprecated GCM and replaced it with Firebase Cloud Messaging Service (FCM) and then shut down GCM completely in 2018.

Timeline so far:
C2DM 2010 -2012
GCM 2012 - 2018
FCM 2014 -

If you ever come up with C2DM in any legacy code, you should replace it with the Firebase Cloud Messaging Service.