By Alan Baldwin

LONDON (Reuters) - Greatness can be hard to define but two words suffice for Olympic swimming - Michael Phelps.

Despite a global cast of world record holders, champions and swim sensations preparing to light up the Rio pool, the greatest of all time will again be the center of attention at his farewell Games.

Phelps, who retired in 2012 before a rethink, may not win the most medals but his fifth Olympics are about more than that.

"I came back because I wanted to," he said at the U.S. trials. "I wanted to do this for me."

The 31-year-old has three individual events (100 meters butterfly, 200 butterfly and 200 individual medley) plus relays with a younger-looking U.S. team that has rung some changes since London.

However big the splash he makes, Phelps is ratings gold as he seeks to add to his record haul of 18 Olympic titles and 22 medals in total.

Winner of the 100 butterfly and 200IM at the past three Olympics, Phelps can become the first swimmer to win the same individual event at four Games in a row.

He is not the oldest U.S. swimmer in Rio, however, with former bad boy and 2000 gold medalist Anthony Ervin back at 35. Ryan Lochte, 31 and an 11 times medalist, will be at his fourth Games.

If anyone is going to carry Phelps' mantle at a meet that will run well past midnight on some days to cater for U.S. viewers, then it is likely to be Katie Ledecky.

The 19-year-old American holds the world record in 400 and 800 meters freestyle and could complete a golden triple with the 200 as well.

Ledecky, who won four individual golds at last year's world championships in Kazan, Russia, is the youngest member of the U.S. squad but looking unbeatable in 800.

Team mate Missy Franklin, who won four golds and a bronze in 2012, will compete in only two individual events and a relay after a disappointing trials.

AUSTRALIAN COMEBACK

The rest of the world, and particularly the Australians, sense an opportunity.

London saw the once-mighty swimming nation slump to its lowest Olympic haul in 20 years, with a subsequent review highlighting a 'toxic culture', but Rio can expect a fighting comeback.

"I think swimming has got its mojo back which is a great thing," said head coach Jacco Verhaeren after national trials in April.

Cate Campbell set a 100m freestyle world record in July while sister Bronte is world champion but both face a challenge from Sweden's Sarah Sjostrom.

On the men's side, Cameron McEvoy will chase gold in the blue riband 100 freestyle, one of five events he will contest.

Golden couple Emily Seebohm and boyfriend Mitch Larkin go to Rio as world champions in both the 100m and 200m backstroke.

The Europeans will get in on the act, with Britain's breaststroke world champion Adam Peaty and 200m free world champion James Guy hoping to succeed for a country that has not had a male Olympic swimming gold medalist since Adrian Moorhouse in 1988.

Hungary, Italy, France - with reigning 50 meters freestyle world and Olympic champion Florent Manaudou - and the Netherlands all have potential medal winners too.

Africa will look to South Africans Cameron van der Burgh, the reigning champion and Peaty's big rival in 100m breaststroke, and Chad Le Clos, who beat Phelps to 200m butterfly gold in 2012.

The scourge of doping, in a year darkened by scandals in Russia, also hangs over the pool with 2012 gold medalists Sun Yang of China and South Korea's Park Tae-hwan back in Rio after suspensions.

Pollution is also a major concern for the open water swimmers after dangerous 'super bacteria' were found off Rio's beaches.

(Reporting by Alan Baldwin, editing by Rex Gowar)