For most of the Compromise of 1850, the author doesn't shy away from the word "slave" or "slavery". The text refers to how states "shall be received into the Union, with or without slavery, as their constitution may prescribe at the time of their admission." (Utah.Section 1.2), and discusses how court cases "involving title to slaves" (Texas.Section 10.10) should be handled.

Even the bill banning the slave trade in D.C. comes right out and says it: "it shall not be lawful to bring into the District of Columbia any slave whatever, for the purpose of being sold." (DC Slave Trade.Section 1.1)

But then, there's the Fugitive Slave Act. Although it isn't technically called that in the actual bill, because throughout this section, they're known as "fugitives."

In the title, runaway slaves are referred to as "Fugitives from Justice, and Persons escaping from the Service of their Masters" (Fugitive Slave Act.Title). The purpose of the law is laid out as "a view to afford reasonable facilities to reclaim fugitives from labor" (Fugitive Slave Act. Section 3.1), and gives slaveowners (AKA "claimants") "authority to take and remove such fugitives from service or labor, under the restrictions herein contained, to the State or Territory from which such persons may have escaped or fled." (Fugitive Slave Act. Section 4.1)

The word "slave" or "slavery" is actually never used in this bill. Why might that be?

Well, it probably has to do with the fact that this was the most controversial part of the Compromise, to which people in the North were very, very opposed. Yet southerners and some northerners did think it was the South's right to have a system in place for fetching runaway slaves, since at this point slavery was considered protected by the Constitution. By describing folks as lawbreakers rather than slaves, maybe northerners could be persuaded to comply.

Runaway slaves aren't escaping injustice; they are people "held to service or labor" (Fugitive Slave Act. Section 6.1) who've abandoned their posts. The "held to service or labor" language happens to be identical to how slaves are referred to in Article IV, Section 2, of the Constitution. And who can argue with the Constitution?