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Open source
In short and in plain language
Open source means the code of a program is open to all.
You may use it, change it and pass it on. The rules of its licence apply.
Often there is no fee to use it, and you are not tied to one provider.
Definition
Open source refers to software whose source code is openly available and which may be used, adapted and passed on under its licence. For small businesses this can mean that no licence fee applies and that they are not tied to a single provider. The terms of the licence still apply.