大股东
美国股权分散，只披露持股5%及以上的股东。
blockholder就是披露的持股5%及以上的股东

一般是持有股份比例比较高的股东，以这篇13年JF的文章为例，exit as governance-an empirical analysis. Blockholders: Parties whose beneficial ownership in the firm exceeds 5%.
Beneficial Ownership: BO is enjoyed by anyone who has the benefits of         
       ownership of a security or property, and yet does not nominally own the                              
       asset itself.

There is not set number of shares to make somebody a blockholder, although the SEC does require any 5% or larger equity owner to file paperwork stating as much.