A MLP is a fully connected feed-forward neural network made up of perceptrons. Fully connected means any given perceptron of any given layer (except the first layer) is connected to all perceptrons of the previous layer. Feed-forward means there's a clear ordering of layers and connections occur only between consecutive layers (i.e. there are no connection cycles or skip connections). Intermediate layers (all except the first and last) are called hidden layers, and its perceptrons are called hidden units.