My knowledge comes from what I've found to be evident in some treatises and manuscripts, and other documents and findings, and how I've come to classify things with my fight group, so I accept the possibility that I could be wrong, but...
The projectile in your first photo is long, thin, and has fletching, and has to be launched with a tool. That constitutes an arrow. A dart is shorter, and doesn't necessarily need a tool to be launched properly. Then you've got javelins, which lack fletching and are thicker than arrows and darts, but function the same as the latter. Lastly you've got spears, which are the thickest of the bunch and also lack fletching, and cannot be thrown properly without being made specifically for the purpose of being thrown.
Using an atlatl doesn't automatically make whatever you're throwing a dart. Atlatls can be used to launch all of the mentioned projectiles if they are made and used correctly.