Retrench vs Undefined - What's the difference?

As a verb retrench

is to cut down or reduce or retrench can be to dig or redig a trench where one already was.

lacking a definition or value.

(etyl) retrenchier, (etyl) retrancher; see (etyl) , and English trench.

(es)

To cut down or reduce.

* Denham

Thy exuberant parts retrench .

To abridge; to curtail.

* Milton

But this thy glory shall be soon retrenched .

To confine; to limit; to restrict.

(Addison)

* I. Taylor

These figures, ought they then to receive a retrenched interpretation?

To furnish with a retrenchment (defensive work within a fortification).

to retrench bastions

To take up a new defensive position.

We must retrench and try to hold on long enough for products in development to reach the market or we will be out of business.

*2012 , The Economist, Private Equity: Keep Calm and Carry On

International firms could decide it is not cost-effective to keep open their other European offices and retrench to London.

(es)

To dig or redig a trench where one already was.

(wikipedia undefined) (-)

Lacking a definition or value.

(mathematics, computing) That does not have a meaning and is thus not assigned an interpretation.

The result of division by zero is undefined .

* defined