Front end

part of a program which is responsible for the user interface

In their most general meanings, the terms front end and back end refer to the initial and the end stages of a process flow. These terms acquire more special meanings in particular areas. The general idea is that the front end is responsible for collecting input from the user, which can be in a variety of forms, and processing it in such a way that it conforms to a specification that the back end can use. The connection of the front end to the back end is a kind of interface.
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Noun
1. the side that is forward or prominent
(synonym) front, forepart
(hypernym) side, face
(hyponym) forefront, head

1. An intermediary computer that does set-up and filtering for another (usually more powerful but less friendly) machine (a "back end").
2. Software that provides an interface to another program "behind" it, which may not be as user-friendly. Probably from analogy with hardware front-ends that interfaced with mainframes.
[Jargon File]

the designated area of a retail store for consumer checkout, payment transactions and bagging stands. Usually operates as a sub-department to the store as a whole.