What we in the industry refer to as, Legs.

Back in the day, films would remain in theaters for several months, showing small drops or gains from week to week, slowly raising their tally to blockbuster numbers. Today, the opening weekend is everything for big films, and only overwhelmingly good word-of-mouth can keep a movie going after a lack-luster start.

Looking back to 1980, the top five films were:

The Empire Strikes Back-$210,000,000, 5.2% from opening weekend.
9 to 5-$103,000,000, 3.8% from opening weekend.
Stir Crazy-$101,000,000, 8.6%, from opening weekend.
Airplane!-$83,000,000, (opening unknown)
Any Which way you Can-$71,000,000, 11.4% from opening weekend.

Now lets look at 2009:

Avatar-$745,000,000, 10.3% from opening weekend
Transformers 2-$402,000,000, 27.1% from opening weekend
Harry Potter 6-$302,000,000, 25.8% from opening weekend
New Moon-$297,000,000, 48.2% from opening weekend
Up-$293,000,000, 23.2% from opening weekend

2009 is probably not the best example of this trend however, as Avatar had shocking longevity, and a holiday release, which always marginalizes the opening weekend. Transformers 2 had a five day opening, meaning the opening day is not included in the opening weekend. Harry Potter 6, also had a five day opening.

These days, films with significant legs are a real rarity. One such film is "How to Train Your Dragon". After superior word of mouth (98% on rotten tomatoes) the movie has maintained a steady stream of cash since its somewhat tepid premiere ($43,000,000), and managed to take #1 this weekend. This is an extremely rare case of a film retaking #1 after falling out, something that hasn't happened in a very long time, perhaps ever for a film in its fourth weekend. It now totals $159,000,000, and will probably cross $200 million by the end of its run.

By comparison, Transformers 2 earned $200 million in its five day opening and struggled to get over the $400 million mark. Word of mouth really does help a film.