A sedative is a drug that calms you down. If a patient is freaking out about getting an MRI or minor surgery, the doctor might prescribe a sedative to chill them out.
A sedative is anything that makes you sedate — peaceful, calm, tranquil. In medicine this word refers to drugs that relieve anxiety or tranquilizers that pretty much knock you out. But other non-pharmaceutical things can have sedative effects, like a soothing cup of tea or a relaxing glass of wine. The tryptophan in turkey is actually a natural sedative — that's why everyone is sleepy after Thanksgiving dinner!