Picture a hall where something is happening right now — a ceremony, a talk, a concert — and one camera at the back sending it out through the internet. Far away, hundreds of people open the same page at the same minute and see the same stage. They are watching, not talking. When the event ends, the thing ends. That single stream going out to many quiet viewers, at a set time, is the whole of this word.
Wiktionary gives the word as web plus -cast, taken from broadcast. The whole meaning sits in that join: broadcast already meant sending one thing out to everybody at once, and web says the route is the internet rather than the air.
字典寫得很直接：web 加上從 broadcast 拿來的 -cast。意思整個就在這個組合裡。broadcast 本來就是「一次送給所有人」，web 只是把管道從空中電波換成網路。所以這個字沒有什麼隱藏的比喻，它的兩半各說一件事：送什麼方式、走哪條路。
Wiktionary, 'webcast' (packet source_ref kaikki:3f6f69a190036ae4:685437255:1927)
One branch
把現場送上網，大家同時看
名詞和動詞是同一個畫面的兩面：一個是被送出去的那場活動，一個是送出去這個動作。
noun · B2
A live event, such as a talk, a concert, a ceremony, or a company meeting, that is sent out over the internet so that people in many places can watch or listen to it while it is happening.
網路直播（節目）
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
webcastnoun
A video and/or audio broadcast transmitted via the Internet
webcastverb
To make such a broadcast
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。