You almost figured it out when you said ‘cocktail.’ The modern term ‘lounge’ may or may not be accurately defined by your average encyclopaedia. Why?
Because ‘Lounge’ has come a long way, beyond the boundaries of ‘Easy Listening’ for uncles and aunties! For modern fans of the 'Downtempo' music genres, their music preference is anything but dull. Rather, it is vibrant (when played on full-sized sound systems) and sultry (and it's not just in the album artwork, a-hem!) and is often described as being 'chilled.'
So what’s ‘loungy’? What’s ‘Downtempo’? What’s ‘Chillout’? I shall give my own version. You can always Wiki/ Google them for the ‘official’ versions….
What’s In the Music.
Lounge and Chillout music, though different to some fans, often end up on similar shelves (in shops and at home.) They are both for ‘winding down’, for relaxing to (but not always, as many types have a ‘dance’ beat.) They can be electronic - or made with real instruments, both Western and ethnic. Often it has Latin American influences (such as Bossa Nova and Tango.) It can also have other cultural influences from around the world, such as Middle Eastern music. And of course you have the more commercially assessable old jazz standards. On one extreme, it could be danceable like Brazilian and Cuban music; and on the other extreme, it could be as relaxing as Yoga/ meditation music.
Downtempo.
Officially the term makes reference to certain sub-genres of Electronica that have a slower tempo. Downtempo is very often associated with Nu Jazz. When more loosely used, Downtempo can be a broad umbrella term often used interchangeably with Chillout/ Lounge. Can ‘Downtempo’ be used too loosely? Well, you can look at your TV and record shops out there and decide.
Downtempo/ Lounge Examples.
Examples of artistes, DJs and electronic bands that capture Down Tempo/ Lounge (in my opinion):
De-Phazz; Thievery Corporation; Ursula 1000; Stephane Pompougnac; Walter Wanderley.
Outside the Lounge Category
Even the term ‘loungy’ can be used outside of its usual Lounge category. Like, a rock band or a jazz/ pop singer (such as Joni Mitchell; or the Flaming Lips) might one day make a ‘loungy’ sounding song/ album that might not be typical of their ‘usual style.’
A striking example of a ‘non-lounge’ artiste whose music can sometimes very loosely be described as ‘loungy’ – is Art Rock singer Bryan Ferry.
Examples of modern Lounge/ Downtempo mix compilations/ labels:
Buddha Bar; Hotel Costes; Bar De Lune Records.
Notes:
Kudos to those who beat me to answering this question. All have valid points. Any similarities are coincidental. My answers intend to satisfy the question (of what makes a song ‘loungy.’) Any explanatory details about ‘Lounge’-affiliated genres are meant for the sake of the readers and are not meant to be comprehensively ‘encyclopaedic.’