Apart from being a major movie rental chain (see writeups above), the word blockbuster has three other meanings;

The earliest use of the word comes from World War II, and was used to describe a large aerial bomb, large enough to lay an entire city block to waste.

Additionally, a blockbuster can mean a real estate agent who sells a house a house in a "white neighborhood" to an unwanted minority, such as a african-american family, thus "busting" the block. As above, the symbolic meaning is that the block is destroyed, as the value of the houses in the neighborhood are expected to fall. This usage of the word is not more or less extinct.

The third and by far most popular sense is something that is large, important or popular, such as a movie, a play, a piece of music or a book.