To download is computer jargon meaning to transfer information (usually one or more files) from a server to a client. In a peer-to-peer or file sharing environment, download often refers to a transfer to your local computer.
A transfer in the reverse direction is called an upload. In a peer-to-peer environment, upload often refers to a transfer from your local computer.
As a noun, a download is the information transferred or the act of transferring it.
In normal usage, a file is typically uploaded or copied to a server once and subsequently downloaded many times by many clients.
In some circumstances, either machine might be considered the "server" or the "client". Generally, the server is the larger computer, to which many smaller clients connect. An exception is used when talking about data transfers between space vehicles and ground stations; in this case, every transfer from the space vehicle is a download, while every transfer to it is an upload.
In surfing the World Wide Web the term downloading usually is not used for simply bringing up a web page, but rather for transferring a larger file and/or directly saving it. In a network, the term downloading usually is not used for copying files from one network device to another even if one computer is acting as a host. (See copy.)
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