To give in is to stop trying to resist something. You agree to something that you had been fighting against. You're not giving in to someone else's desires, but making your own decision to quit.
After many months of resisting, Mr Nathan finally decided to give in and have a swimming pool built in the garden; this made the children very happy.
I don't know where we will spend our holidays, as both my family and my wife's want us to go to their homes, and neither side seems willing to give in.
Another similar phrase is to give up. But it has a slightly different meaning, because to give up is to admit that you cannot do something and stop trying, to quit; or to stop having, doing, or using something.
Marcia says she's tried so hard to please her boss over the past year and it's just not working; she's ready to give up and look for another job.
Mrs. Harrison told us that, due to her age, she's going to have to give up gardening. She just can't take all the kneeling and up-and-down movements it requires.
Defeat is failure to succeed or to win in some competition or battle. To admit defeat is to agree that you are beaten or that you have not succeeded in what you would like to do. A politician admits defeat when the election results show that he has lost. Sometimes it means that you agree you can't do something alone and are ready to get some help.
After struggling with biology class all year, Theodore had to admit defeat and recognize he was not meant to be a doctor.
As Penelope works from home while raising three children, sometimes she has to admit defeat and be satisfied with her house being less clean and organized than she would like.
Defeatism is a way of thinking in which a person always expects to fail. If you give up applying for jobs because no one is calling you, that's defeatism. A person who thinks that way is a defeatist.
I don't want to hear you say that you can't succeed at school, Anna. Such defeatism does not do you any good; try to think positively.
Jeremy is such a defeatist. He was sure he had failed the exam before he even finished it. And when he got the news that he had passed, he found it hard to believe.