Jupyter is an abbreviation for Julia, Python, and R, the three languages it was originally designed to support (Jupyter notebooks can now be used for many languages). Project Jupyter refers to its notebooks as "computational notebooks".
"Jupyter Notebook" can refer either to these computational notebooks or to the specific Jupyter Notebook platform for authoring them. Jupyter Notebooks allow code and text to be presented elegantly in the same document, they are a popular format for data analysis and data science. Jupyter Notebook and JupyterLab are both platforms that support these computational notebooks (.ipynb or "IPython notebook" files).
JupyterLab is a notebook authoring application that provides more features than its older sibling, Jupyter Notebook. Among these features is the ability to work with two or more notebooks in the same window, allowing for a smoother workflow across projects. Though both Jupyter Notebook and JupyterLab are available side-by-side, this will not always be the case, as JupyterLab will eventually replace Jupyter Notebook in future versions.
