Dear @maitatoy, thank you for your question. I think that it is something that a lot of people wonder when they question if they/someone they know are depressed.
Based on personal experiences, the difference between depression and sadness is whether you can function in your daily life despite feeling it. I think that you can still feel sad for no reason, or that the reason is so deep seated that it is your subconscious leading you to feel sad. Similarly, people can slip into depression due to a trigger in life, or simply because they woke up today feeling depressed.
Depression is defined as having the symptoms for 2 weeks and above. I think the defining difference between depression and sadness is that if your symptoms persists for 2 weeks and above, it means that it has a major impact on your life and how you function daily, your work/studies, your interpersonal relationships and your health (such as sleeping patterns/insomnia/bingeing).
If you have depression (symptoms for more than 2 weeks), it would mean a major impact on your life. Two weeks of leave from school or not doing your work well is half a month of suffering already. That is not counting the amount of time people take to seek help, and the time before any therapy or medication can have its effect. I believe that when you are sad, something outside of the sad circumstances can make you feel better. However, with depression, it often seems that nothing helps, and you need to seek professional help in the form of therapy or medication, as it is a biological cause with neurotransmitters involved, just like any physical ailments.
Have questions on Mental Health? You can ask them in this forum or share your personal experiences. The information shared in the forum is not meant to substitute professional medical advice. It is solely for educational purposes. If you suspect that you are experiencing a mental health condition, please seek professional help.