A button-up shirt is any shirt that buttons up the front, including dress shirts. It has buttons running down the front, from the collar to the hem. It is a versatile piece of clothing that can be worn for both formal occasions and casual outings.

A button-down shirt specifically refers to a shirt where the collar has buttons that fasten it down to the shirt. It's a bit more casual and great for both professional and relaxed settings.

Crew-neck is a term used to describe the round neckline on garments such as T-shirts, sweaters, and other tops. 

The term "v-neck" refers to a style of neckline that dips down in the front in a V-shape. 

在挑選上衣時，了解不同的領口和鈕扣設計可以幫助你找到最適合自己的風格。以下這些單字常用來描述襯衫或T恤的款式，是日常穿搭中很實用的詞彙：

• Button-up shirt（全扣式襯衫）：前面有整排鈕扣，從領口一路扣到下擺，是正式或半正式場合常見的款式。
  He wore a white button-up shirt to the interview.

• Button-down shirt（扣領襯衫）：這是button-up的一種，特色是領子有小鈕扣固定，讓領子不會亂動，看起來更整齊。
  His button-down shirt made him look a bit more formal.

• Crew neck（圓領）：領口是圓形且貼近脖子的設計，常見於T恤或毛衣，是最基本的休閒款式。
  I prefer crew neck T-shirts because they feel more casual.

• V-neck（V領）：領口呈V字形，讓脖子看起來比較修長，也比較透氣，適合想穿得輕鬆又有點造型感的人。
  She wore a black V-neck sweater with a necklace.

"Button up" is a phrase we use when talking about fastening the buttons on a piece of clothing, usually a shirt, jacket, or coat. For example, if it's cold outside, someone might say, "It's freezing! Make sure to button up your coat." It can also be a command, like "Button up your shirt before the meeting!" This phrase is mostly used when someone is putting on or adjusting their clothes to look neat or stay warm.

「button up」這個片語最基本的意思是把衣服的鈕扣全部扣好，常用在天氣變冷或要保持體面時提醒別人，例如："It's getting cold, so button up before you go outside." 此外，它也可以引申為「把事情收尾、確定所有細節都完成」的概念，像是："We should button up this report before the deadline." 在更口語的場合，有時也會被用來叫人閉嘴，不過這層用法比較少見。整體來說，看到或聽到「button up」，你可以先判斷是在談衣服還是事情的完成度，再決定正確的中文意思。