A smack is a slap or blow made with the palm of a hand or something else that's flat. Instead of giving your dog a smack for chasing the cat, you should speak calmly and firmly to him.
Smack also describes the sound of a smack, like when your Frisbee hits your neighbor's house with a smack. It's a verb too: "I'm tempted to smack you for saying that about my brother." Sometimes, smack takes the form of a descriptive adverb: "I ran smack into the mailbox while I was texting and walking." Informally, a smack can also be a kiss, or an eager movement of the lips in anticipation of something delicious.