Same origin policy

依據 Wikipedia [1] 上對於 Same origin policy 的描述如下：

In computing, the same origin policy is an important security concept for a number of browser-side programming languages, such as JavaScript. The policy permits scripts running on pages originating from the same site – a combination of scheme, hostname, and port number[1] – to access each other's methods and properties with no specific restrictions, but prevents access to most methods and properties across pages on different sites.

簡單來說，大概就是為了安全性的考量，要限制瀏覽器端的程式語言只能夠存取相同網域內的資源。
另外節錄在 [1] 中對於同網域的認定範例：

Compared URL	Outcome	Reason
httpː//www.example.com/dir/page.html	Success	Same protocol and host
httpː//www.example.com/dir2/other.html	Success	Same protocol and host
httpː//www.example.com:81/dir/other.html	Failure	Same protocol and host but different port
https://www.example.com/dir/other.html	Failure	Different protocol
http://en.example.com/dir/other.html	Failure	Different host
http://example.com/dir/other.html	Failure	Different host (exact match required)
http://v2.www.example.com/dir/other.html	Failure	Different host (exact match required)

參考資料：
1、Wikipedia: Same origin policy
2、Cross Domain Ajax 跨網域抓取資料(JSONP)