The definition of discipline is to train someone to follow rules, usually with rewards and punishments. As an educator and mother, discipline to me means teaching someone to do the right thing because it is the right thing to do. It involves teaching the child to have self discipline. When children are very young that is a big goal. When a child is old enough to understand what the word "No." means, you can begin teaching them to do the right thing. Usually 18-24 months of age. However, punishment and reward is not usually the way to go with a child this young. I found with my own children, distraction worked better. If they are doing something they shouldn't because it's not safe, remove them from the situation or give them something else to focus on. If there is something you don't want the child playing with, then put it out of their reach. Why create a battle when you can just avoid the battle?
Time out still works great once the child is old enough. Usually around 4 years of age. They should only spend 1 minute per year of age so about 4 minutes for a 4 year old. They should be told why they are there, "To think about what they did and what a better choice would have been.", they need to stay in the designated place and remain quiet the entire time. If they are screaming and yelling or getting out of the spot, the timer does not start until they comply. Do not engage them in any way. Do not yell or argue with them. Their goal (whether they know it or not) is to get attention from you, if you begin yelling or negotiating you have lost control of the situation. Walk away. Once it's over, calmly discuss what they did, what they should do next time and move on. Don't force them to apologize if they are not really sorry. Asking them to apologize when they are not sorry is teaching them to lie.
I'm not a fan of spanking. Having said that, I am not going to say it should never be done. As a parent, and a teacher, I know there are a few situations where it may be beneficial to the child. Spanking is NOT beating. It shouldn't leave marks or break the skin. It shouldn't be excessive in duration and things like straps should not be used. A spanking should never be done in public or to embarrass the child. It should be a few (1-5 swats) on the bottom, when behavior is dangerous or other methods have failed. Never before age 4 and not because you are angry. It should also be done on the spot when possible. Immediate consequences are best.