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If someone tells another person to “keep in touch,” it means that he or she wants the other person to stay in contact. This phrase, which is an English idiom, is often used right before someone makes a trip that will take him or her far away from someone else. As a result, it is often the last thing uttered as a reminder to the person taking the trip that the other person wants to keep up communications even over the long distance. “Keep in touch” can easily be replaced by the similar phrase “stay in touch” as a way to urge someone to maintain contact.
Whenever someone uses a word or phrase with a meaning that is somewhat different from its literal definition, he or she is using an idiom. Idioms are useful constructions in the English language in that they provide a way for people to speak in a colloquial manner with impact and color. These idioms generally evolve over time to gain meanings that are separate from what they might have meant when they were first used. One idiom used on someone who is about to leave is the expression “keep in touch.”
This phrase is often used when someone, usually a loved one or a close friend of the speaker, is about to depart on a long journey. As a result, communication won’t be available through direct conversations. The person using the phrase wants the person departing to make the effort to keep communication lines open through telephone conversations, letters, or whatever means are available. For example, someone might say, “I hope that you keep in touch when you go away to college so that your father and I know how you’re doing.”
It’s also common for this phrase to be used all by itself as a stand-alone part of a sentence. In this way, it is almost like a term of endearment or a heartfelt farewell said by one person to another who is about to leave. As an example, someone might say, “I’m going to miss you; keep in touch.”
The other idiom often associated with “keep in touch” is the phrase “stay in touch.” Both phrases mean essentially the same thing, and they both get their meaning from the fact that, figuratively, someone “in touch” with someone else is in contact with him or her. Thus, this phrase implores a person leaving to keep that contact even over the miles that separate the two people in question.
It can also mean using some sort of technology to facilitate keeping in touch, like Facebook or the like. "Keep in touch" can mean something as involved as chatting with someone on Chat Rounds regularly to just leaving a few "Likes" on someone's status update. It's a really flexible term.