<p>The short answer is NO, clustering is not the only field under unsupervised learning. Unsupervised Learning is way more broader than only clustering. Clustering is just a sub-field of (or type of) unsupervised learning.</p>
<p>Little correction: KNN is not a clustering method, it is a classification algorithm. You probably meant to say k-means.</p>
<p>The essence of unsupervised learning is basically learning data without ground truth labels. Thus, the goal of unsupervised learning is to find representations of data given. The applications of unupervised learning vary a lot, though academically it is true that the field is less attractive to researchers due to its complexity and effort to build new stuff and/or make improvements.</p>
<p><a href="https://en.wikipedia.org/wiki/Dimensionality_reduction" rel="nofollow noreferrer">Dimension reduction</a> can be considered under unsupervised learning as you want to find a good representation of data in lower dimensions. They are also useful for visualizing high-dimension data. PCA, SNE, tSNE, Isomap, etc. are type of these applications.</p>
<p><a href="https://en.wikipedia.org/wiki/Cluster_analysis" rel="nofollow noreferrer">Clustering</a> methods are type of unsupervised learning as well where you want to group and label values based on some distance/divergence measure. Some applications could be K-means, Hierarchical clustering, etc.</p>
<p><a href="https://en.wikipedia.org/wiki/Generative_model" rel="nofollow noreferrer">Generative models</a>, generative models model the conditional probability P(X|Y=y). The research in this field boomed since the publication of <a href="https://en.wikipedia.org/wiki/Generative_adversarial_network" rel="nofollow noreferrer">GAN</a> (see <a href="https://arxiv.org/abs/1406.2661" rel="nofollow noreferrer">paper</a>). GANs can learn the data distribution without seeing the data explicitly. Methods are various where GANs, VAE, Gaussian Mixture, LDA, Hidden Markov model.</p>
<p>You can read further <a href="https://en.wikipedia.org/wiki/Unsupervised_learning" rel="nofollow noreferrer">here</a> on unsupervised learning.</p>
Unsupervised Classification in Machine Learning
<p>Clustering (eg: K-means , EM algorithm etc) is used for unsupervised classification by forming clusters in the data sets using some distance measurement between data points</p>
<p>My question is :
Other than clustering what can I use to perform unsupervised classification and how? Or is there no other option other than clustering for unsupervised Classification?</p>
<p>Edit: Yes I meant k-means</p>