Yes, yam is a Scrabble word!

Noun

1. Edible tuber of any of several yams

2. Any of a number of tropical vines of the genus Dioscorea many having edible tuberous roots

3. Sweet potato with deep orange flesh that remains moist when baked

4. Edible tuberous root of various yam plants of the genus Dioscorea grown in the tropics world-wide for food