'Plunge' if it refers to perpendicular cutting into a material (e.g. plunge milling or routing), 'penetration' if it refers to the entry of the cutting tool (edge) into the material. (although I have the vague idea I've also seen the term used for lathe tools).

sample refs:

End Milling Cutter provides light depth of cut., Carboloy Inc., A ...
For Lighter Depth-Of-Cut On Small Machines Carboloy Introduces Nano TurboMillTM, ... Apr 29, 2004 - Plunge Milling Cutter is designed for metal removal. ...
news.thomasnet.com/fullstory/6838

High Performance Milling Cutters
When ramping and plunging, the inserts cut with their inside and outside edges at the ... Milling to a specified depth, in this case, will require a greater ...

Scanning the Horizon - Tools For Milling In Z
Instead, the following examples of Z-axis milling cutters represent a range ... The Axial Feed Milling System includes tools for plunge milling along edges ...
www.mmsonline.com/articles/0201scan1.html

Software
Wiederholungsfunktion beim Fräsen mit automatischer Höheneinstellung. - Getrennte Geschwindigkeit der Z-Achse für langsames Eintauchen des Fräsers ...
www.easgmbh.de/cnc/soft.htm

Werkzeugwege Fräsen Level 3
Werkzeuge Fräsen Level 3. Klicken Sie auf ein Bild, um eine vergrößerte Darstellung ... Einfahrmethoden für sanftes, effizientes Eintauchen in das Material ...
www.mastercam.de/wzwegefrl3.htm


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Note added at 2 days 12 hrs 0 min (2005-06-19 22:12:44 GMT)
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That should read \'The most common refs are for milling, but I have the vague idea...