The linear model is a terminology that is usually used in statistics. It can be used in several ways, depending on the context. When we are talking or a linear model, it is a model whereby terms are added, rather than divided, multiplied, or obtained as a non-algebraic function. A linear model does not compulsorily have to be a straight line or its analog when it comes to higher dimensionality.
Sometimes, a curved model can still be referred to as linear. This comes up from particular mathematical connotations, but might seem oxymoronic. Linear models are usually created with the use of a statistical method called linear regression.
However, when it comes to communication, the linear model refers to a kind of one-way process whereby a sender sends a message, and there was no response or feedback from the message receiver. The signal of the message is encoded and then transmitted through a channel with noise.