Glossary of Terms

An alphabetical list of common GD&T terms with simple definitions for quick reference.

A–D

Angularity
Orientation tolerance controlling a feature at a specified angle other than 0° or 90°.

Axis
The centerline of a feature (e.g., a hole or shaft).

Basic Dimension
A theoretically exact value (boxed) used with GD&T.

Bonus Tolerance
Extra tolerance gained when a feature departs from MMC or LMC.

Circularity (Roundness)
Form tolerance controlling how close a cross-section is to a perfect circle.

Circular Runout
Runout measured at a single cross-section during rotation.

Concentricity
Controls alignment of median points of a feature to a datum axis.

Cylindricity
Controls the overall form of a cylindrical surface.

Datum
A reference point, axis, or plane used for measurement.

Datum Feature
The actual physical feature used to establish a datum.

E–L

Envelope Principle (Rule #1)
Perfect form is required at MMC.

Feature Control Frame (FCF)
A box that defines GD&T requirements (symbol, tolerance, datums).

Flatness
Controls how flat a surface is.

GD&T (Geometric Dimensioning and Tolerancing)
A system for defining and controlling geometry in engineering.

Least Material Condition (LMC)
Condition with the least amount of material.

Limit Dimensions
Dimensions expressed as upper and lower limits.

M–P

Maximum Material Condition (MMC)
Condition with the most material.

Parallelism
Controls parallel orientation relative to a datum.

Perpendicularity
Controls 90° orientation relative to a datum.

Position
Controls the location of a feature relative to datums.

Profile of a Line
Controls a 2D cross-sectional shape.

Profile of a Surface
Controls a 3D surface shape.

R–T

Regardless of Feature Size (RFS)
Default condition where tolerance is independent of size.

Runout
Controls variation of a feature during rotation.

Straightness
Controls how straight a line or axis is.

Symmetry
Controls equal distribution about a datum plane.

Tolerance Zone
The allowable region where a feature may vary.

Total Runout
Controls overall variation across the entire rotating surface.