The placebo effect is that wonderful thing, a scientific principle that's actually understandable. The idea is that if you give a poorly person something he thinks is medicine, then his confidence in the stuff will make him expect a recovery, and so hasten it.
It sounds a bit unethical, but it's not really any different from an old-fashioned cataplasm, or kissing someone better.
The placebo effect is so well-established as a scientific principle that it has to be taken into account when new medicines are tested. You have to get a bunch of people with the same illness, give one group the medicine and the others stuff which contains no medicine, without telling anyone which group is which. Then you look to see if one lot get better more quickly than the other.
What's sometimes called a placebo can also be tried on angry or grumpy people in order to soothe them/make them happy/make them rewrite his or her Will. A placebo here might take the form of a chocolate, a luxury holiday for two on a tropical island (sadly this is all too rare), or a nice cup of tea.
If you're a Roman Catholic then a placebo may be the evening service of the offices of the dead.
That sounds a bit surprising, but there actually is a link: the clue's in the Roman bit.
Word To Use Today: placebo. The Catholic word came into English first, in the 1200s, because the first words of the vespers of the offices of the dead are Placebo Domino. Placebo means I shall please (and Domino means the Lord).