ballast

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ballast

(bal'ăst),
1. Material of doubtful value itself used to stabilize something (for example, maritime practice). 2. Something largely without intrinsic value.

ballast 

Additional weight of material incorporated in a part of a contact lens to maintain it in a given orientation (Fig. B1). This is often provided by giving prismatic power to the lens (prism ballast lens).
Fig. B1 Truncated and prism-ballasted toric contact lensenlarge picture
Fig. B1 Truncated and prism-ballasted toric contact lens