Vanishing
Part of speech: verb
Definition: To become invisible or to move out of view unnoticed.
Disappearing
Part of speech: verb
Definition: To vanish.
Example sentence: Sending a handwritten letter is becoming such an anomaly. It's disappearing. My mom is the only one who still writes me letters. And there's something visceral about opening a letter - I see her on the page. I see her in her handwriting.