In season 1, episode 18, around 4:40 timestamp, there's a piano song in the background whilst Suguha is changing and placing the device on her head.
What is the title of the song?
That song is the the piano only version of "Is this love?" by Kajiura Yuki.
It's the 18th track on the Sword Art Online Original Soundtrack vol.2 which was bundled with the limited edition SAO vol. 7 DVD and Blu-ray.
Source, you can also listen to the song there.