meaning of "defending champions"

[learning_english]

Opener Matthew Hayden scored 41 and Michael Clarke smashed 60 not out to lead the defending champions to victory with 18.3 overs to spare, after their bowlers bundled out South Africa out for a paltry 149 in the second semi-final at the Beausejour Stadium in St Lucia on Wednesday.

What does `defending champions' mean in this context?

Please explain it.
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Please let me know if I have made any mistakes in any of my sentences.

[Mister Micawber]

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Matthew and Michael's team, last year's champions, who are now defending that title this year.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber