You're not describing a concrete thing. Knowing if what you feel corresponds to what others feel when they use the same words runs straight into the heart of the "other minds problem." It's a philosophical pickle.
Meanwhile, on the academic, clinical or neurological side usage of terms changes over time. They also run into the same problem. There are broad ranges of symptoms to these things, whether they are physical-ish (like mild flue) or mental-ish (like apathy).
So, I suggest embracing ambiguity. You just can't one word that describes emotional states or conditions. This is true for a lot of things. Consider "anger." Seems relatively concrete. OTOH, we use it to describe a state of adrenaline fueled rage like you would feel in a fistfight. We use it to describe a long term attitude towards someone. My friend betrayed me 5 years ago. I'm still angry. You're not necessarily feeling the same thing.
Add an "-ish" to the end of these concepts. It takes a whole conversation to understand someone's depression (or other states) with any kind of precision. One word only gets you a part of the way.
I'm not sure where I heard this, but likely it's one of the following podcasts, which are worthwhile.
Sam Harris #91 - The Biology of Good and Evil https://samharris.org/podcasts/91-biology-good-evil/ YT version: https://www.youtube.com/watch?v=rLWzMH8psFU
Peter Attia #51 – Robert Sapolsky, Ph.D.: The pervasive effect of stress – is it killing you? https://peterattiamd.com/robertsapolsky/
Apathy, in my case, felt like the flu. I didn't have energy, didn't want to do anything, felt weak and soulless.
This is not the official definition, but it's a workable description that also points to a solution - process the apathy and then the underlying emotions and the depression should lift. That's what the app I built helps with. But as usual, YMMV.
I'm sure it varies a lot from person-to-person, but for me, I have a constant and profound feeling of sadness and futility, I often feel like crying, and occasionally consider ending my life (I almost fantacise about it, because I wouldn't have to feel this way any more).
One of my parents has manic depression, and a sibling also has clinical depression - I presume there is a genetic factor at play; I was brought up in a middle-class household with parents that loved me, and I seldom wanted for anything. Now, I'm a successful software architect with a wife, kids and side-business. I own my own home, I have 2 nice cars, and I have a high income. Yet I still feel that sadness, and often just don't see the point of living, or life at all, really.
Except for randoms on the internet, I've never told anyone about this. I've come close many times, but I guess the longer you keep a secret, the harder it is to let it out.
If anyone else is reading this, I'd enourage them not to be me, and to seek medical help. If you don't think you can speak to a doctor, I'd also recommend trying 5-HTP - I started 200mg 2x a day a few years back, and the change was incredible. It may have saved my life.
Even considering just that axis, I'm not sure how one would define a spectrum that has low-grade symptoms on one end and manic episodes on the other. If anything there is a spectrum of affective states ranging from severe depression to severe mania, with most people at most times clustered around the origin point - and even at that, you can't map it to a line with two ends because mixed states exist.
Speaking to your specific case, and also for anyone who might be in the same shoes: suicidal ideation isn't the sole purview of depression or even of mood disorders as a whole, and I really wish the conversation around mental health would expand enough for more people (both patients and healthcare providers) to actually tackle it properly. Besides that I don't think you need to encourage anyone "not to be [you]"; you've found something that works for you, which is more than can be said of the healthcare system sometimes.
How would you actually do that? Is there some kind of home test available?
This sound like good advice, but I've kept it hidden so long I'm genuinely afraid that I'll have some kind of breakdown if I let it out.
Except now, I think what is most likely the case is that we use/learned to use words like depression in a context based fashion, like mapping a set of contextual behaviors to a word. The actual experience of depression is sort of non-existent. The usage of the word and the contexts in which it is applicable changes as society changes its interpretation of emotional health and the behaviors or mannerisms that go with it. I think how we use depression in popular culture isn't indicative of anything fixed, it depends on the zeitgeist in a way. I'm sure this is different from a clinical standpoint, I'm not at all qualified to comment on that. But then again, most people don't use it in clinical terms anyway. Still, I think this is distinctly different from 'sad', as in, the context is different for the two states.
The book 'How emotions are made' by Lisa Feldman Barrett might be interesting to you, it goes exactly into this sort of phenomenon.
It's great that more people are talking about their mental health, and that it's becoming more acceptable to seek help. But the word "depression" has become so murky to me, I rarely know what people mean when they use it.
Maybe I just don't understand the word...