In tissue culture, a flask of cells is confluent when the cells cover 90-100% of the growing surface.
This means the cells are ready for passage.
This is determined by simply looking at the flask under a microscope and estimating what % of the surafce is covered by cells.

Con"flu*ent a. [L. confluens, -entis, p. pr. of confluere, -fluxum; con- + fluere to flow. See Fluent.]

1.

Flowing together; meeting in their course; running one into another.

These confluent steams make some great river's head.
Blackmore.

2. (Bot.)

Blended into one; growing together, so as to obliterate all distinction.

3. (Med.)

1. Running together or uniting, as pimples or pustules.
2. Characterized by having the pustules, etc., run together or unite, so as to cover the surface; as, confluent smallpox.
   Dunglison.

 

© Webster 1913.

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Con"flu*ent, n.

1.

A small steam which flows into a large one.

2.

The place of meeting of steams, currents, etc.

[Obs.]
Holland.

 

© Webster 1913.