This is simply how Americans spell **vigour**. Same word, same meaning, same sound — only the letter u is dropped. It means strong healthy energy: the energy of a body that is working well, or the energy a person puts into doing something. You can say a plant grows with vigor, an old man is still full of vigor, or someone shakes your hand with vigor. It is a slightly formal, written-feeling word; in daily speech people are more likely to say energy or strength. American books, newspapers and websites use vigor; British ones use vigour. Australia, New Zealand and Ireland follow the British side. Both are correct — the only mistake is mixing the two spellings in one piece of writing. Note that the adjective loses the u on BOTH sides of the ocean: vigorous, vigorously, never vigourous.