"Modesty" is very subjective, many descriptions simply being a list of what kind of clothing is appropriate and what is not. My description of modesty is simply this: trying not to draw undue attention to yourself. Now, you'll notice that my definition doesn't only apply to clothing. And that brings me to the most important point of this post, in my opinion: Modesty is a part of your character, and your heart determines how modestly you behave, speak, carry yourself and dress.
Merriam and Webster's dictionary defines "modesty" as:
1 : freedom from conceit or vanity2 : propriety in dress, speech, or conduct
And "propriety" is defined as "the state or quality of being proper."
Trying to conduct myself modestly is much more important to me than trying to dress modestly (but of course, I try to dress appropriately as well). I have seen women before wearing things that I would personally consider immodest, but conducting themselves in such a gentle ladylike and "proper" way that it mostly redeemed their appearance to me (I wouldn't count on that working as well for a man, though). For me, this means striving for the "imperishable beauty of a gentle and quiet spirit" first (I Peter 3:3-4) and choosing modest clothes second. You can have strict rules, cover yourself from head to foot and still behave immodestly.
For modesty in clothing, I think it's harder to draw an exact "line." It may seem hypocritical at first glance, but my opinion of what is modest is based on what the other people are wearing at a particular venue. That's what I mean by not drawing undue attention to oneself. As an example, for competitive swimming, a modest suit for me means a one-piece practice suit with more coverage than most; for recreational swimming, it means a one-piece or tankini that fully covers my stomach, and isn't low cut. This is modest at the pool or beach because it covers more than a bikini. However, at a restaurant, my bathing suits would not be modest. (Also, when I'm not actually in the water, I'll usually wear shorts and a t-shirt or a cover-up dress over my suit.)
This makes modesty much more interpretive because rather than having a list of rules, you are just making sure not to draw inappropriate attention. At a fancy dance where everyone would be wearing prom-style dresses, I might wear a strapless dress (not a low-cut one, though) whereas I wouldn't want to wear a strapless top to go shopping. Should I happen to visit a country where the women all cover themselves from head to toe, I might try to do the same (I don't know how hard it might be to find the right articles of clothing, though), but here in America, that would draw more attention than wearing a pair of not-skin-tight jeans or a simple, modest-length skirt and a nice top.
Now, I certainly don't mean to say that my definition of modest dress is right for everyone. I just wanted to point out the relativity that I see in dressing modestly. I like to get my dad's opinion, and if he doesn't approve, I go change. I'm also trying to enlist my brothers' opinions, now that they're older. Erring on the side of modesty would always be better than risking immodesty, and the most important thing for Christians is to bring glory and honor to God through every aspect of our lives. It is not bringing glory to God if I'm wearing something "cute" that's causing my brothers in Christ to stumble. What you might think of as "cute" might look too tempting to your brothers in Christ, and while being tempting and enticing might be "in," that's definitely NOT what Christian young ladies should be aiming for in their dress OR behavior!
"...women should adorn themselves in respectable apparel, with modesty and self-control, not with braided hair and gold or pearls or costly attire, but with what is proper for women who profess godliness—with good works." I Timothy 2:9-10