-tude

turns an adjective into a noun naming a state or quality

Spelled -tude · -tudo, stem -tudin-, a suffix forming nouns of state or quality

Latin -tudo names a condition: altitude measures being high, solitude means being alone, and gratitude expresses being thankful.

Words built from it

altitude — height above sea level or another reference point
Latin altus (high) + -tudo (state or quality) → altitudo (height)
latitude — distance north or south of the equator; also, freedom of choice
Latin latus (broad) + -tudo (state or quality) → latitudo (breadth)
longitude — distance east or west of the prime meridian, measured in degrees
Latin longus (long) + -tudo (state or quality) → longitudo (length)
gratitude — thankfulness
Latin gratus (thankful, pleasing) + -tudo (state or quality) → Medieval Latin gratitudo
solitude — the state of being alone
Latin solus (alone) + -tudo (state or quality) → solitudo
fortitude — courage and strength during difficulty
Latin fortis (strong) + -tudo (state or quality) → fortitudo
magnitude — great size, extent, or importance
Latin magnus (great) + -tudo (state or quality) → magnitudo
aptitude — a natural ability or capacity for learning something
Latin aptus (fit, suitable) + -tudo (state or quality) → Late Latin aptitudo

etude — Ends in tude, but comes from French etude (a study), from Latin studium (zeal, application); those letters are not the suffix.