I was writing my work, I was on the part where I needed to describe the character clothes.
This character wear a skirt and I remember there is this short, usually black, pant (?) that woman wear underneath their skirt that help them moving better when wearing skirt. I forgot the name? It was ‘spart’ or ‘spat’ or something I forgot. Does anyone know?
Do you mean a “skort”? That’s a fairly recent thing, so if the story is set at all in the past I would say something like “short leggings” or “trews”.
They’re known as “spats” but I wouldn’t say it is a commonly-known clothing item. The term “spats” also refers to “splatterdashes” which are guards for boots that cover the instep and ankle, so it can be confusing; although splatterdashes are not really worn anymore outside of the military or steampunk costumes.
In the US, these “spats” types of shorts are known as “spandex shorts” or sometimes “Spanx” shorts (which is a brand name). You could also called them “bicycle shorts” or “yoga shorts” and most people would understand what you mean.
In Australia, you would actually probably have to explain them to most people.
The term “spats” or “spats shorts” would be understood by any otaku worldwide though, since they are very common in anime and manga.
You could even use spanx 
Dead on! 
“Spats” are also known as “compression shorts” in the US, as well.
I… think… 
On a side note, I googled “spandex shorts,” out of curiosity, and ventured into the “images” tab.
A “volleyball” image suggestion tab appeared near the top of the list of images.
More curiosity was stirred, so I clicked on the tab.
I should have known, judging from the images on the suggestion tab, that I was walking into some facepalm-worthy material.
What I saw is proof that Google is still a man’s search engine. 
I’m going to go wash my eyes out.
I’m sticking with Bing from now on. 