[software] QGIS (formerly Quantum GIS) is a free and open source software package that allows you to create, edit, visualize, analyze and publish geospatial information.

[remote sensing] Quad-Trees are raster data structures based on successive reduction of homogeneous cells for the purpose of reducing storage requirements.

[cartography] Quantile classification is a classification method that divides classes so that the total number of features in each class is approximately the same.

[programming] A query is a request or search of spatial or tabular data based on user-define criteria. For example, the results of a query is a subset or selected records. SQL, Python and Java are programming languages how queries are made.