Memory decay refers to the gradual loss of information over time when it is not actively recalled or used. This phenomenon occurs because the neural connections that form memories can weaken if they are not reinforced through repetition or retrieval. There are two main types of memory decay: decay theory, which suggests that memories fade due to the passage of time, and interference theory, which posits that new information can disrupt the retrieval of older memories. Both theories highlight the importance of regular practice and review in maintaining strong memory retention.