Waiting can be tricky to teach! Approaches to teaching this concept can depend a lot on age and typically children require lots of visuals, modeling, and repetition of the word (and its meaning) incorporated into their daily routines.
For example, during play-time, a parent can teach “waiting” by using toys (i.e. toy car/ race track/slide). Place the car at the top of the slide/track and model the word “wait...wait” by saying it to the car and hold up your hand or finger to gesture “waiting". Next, use counting 3..2...1..! This provides a time frame in which the child (car) needs to wait. After counting, release the car and “go, go, go!”
You can do this with multiple toys. Even during bathtime, he/she can make the shampoo “wait” until the child’s hair is wet before opening the top, or a bath toy can “wait” on the bathroom counter before putting them in the water. For example, Mom/Dad could set a timer on their phone so the child has visual and auditory feedback when the timer goes off, or just use a standard sand timer (no electronics)! Allow the child to push the button or flip the timer over and tell the object to “wait”. Once the timer runs out then it’s in the bathwater they go!
There are many ways to make waiting "fun" for your child instead of making it seem like a punishment. Oftentimes it benefits the child to know why they are waiting and explaining it to them helps them to understand and with the consistency, they will become accustomed to it.