Habituation

Habituation is the decrease in the strength of a response to a stimulus that has been presented repeatedly. In other words, when a stimulus is presented to you for the first time you will notice and might even be startled by this new thing, for example when you get a new clock you may notice that it is quite loud, but over time you'll stop noticing the noise. This is habituation. Because the sound of the clock poses no threat to you your unconscious pushes it to the back of your mind and you don't think about it.