Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces

Packages describing “debug” as local USE flag

Package “debug” Flag Description
app-portage/eix Build with upstream's CXXFLAGS/LDFLAGS for debugging support; not recommended for normal use.
dev-haskell/atomic-primops Enable extra internal checks.
x11-misc/albert Enable debugging informations
net-misc/ptpd Enable debugging informations
app-editors/nano Enable debug messages and assert warnings. Note that these will all be sent straight to stderr rather than some logging facility.
sys-auth/pambase Enable debug information logging on syslog(3) for all the modules supporting this in the system authentication and system login stacks.
sys-block/parted Enable debugging as encouraged by upstream: [The default configuration] includes --enable-debug (by default), which contains many assertions. Obviously, these "waste" space, but in the past, they have caught potentially dangerous bugs before they would have done damage, so we think it's worth it. Also, it means we get more bug reports ;)
app-emulation/ski enable extra binary emulation debugging via sys-libs/binutils-libs
app-arch/innoextract Enable debug logging and the --debug command-line option
app-misc/rlwrap Enable debug
app-cdr/dvdisaster Enable memory debugging.
dev-lang/spidermonkey Enable assertions to allow for easier debugging of programs that link to spidermonkey -- note this will often crash software on regular end-user systems
sys-process/fcron Enable debug code and output. Since version 3.0.5 this will no longer force foreground execution, and fcron will be able to run as a service properly.
dev-tcltk/expect-lite pull in packages needed for runtime interactive debugger
net-vpn/wireguard-modules Enable verbose debug reporting in dmesg of various WireGuard peer and device information.
dev-lang/crystal Build each module as a separate object file. Speeds build up.
dev-libs/libevent Support for running in debug mode
dev-perl/perl-headers Add source locations in generated .ph files to assist with debugging when errors occur
app-i18n/librime Enable debugging (including logging of input of RIME input methods)
games-action/dxx-rebirth Enable various debugging code paths and sanity checks, not just output. Some of these checks date back to the retail build, and choke on some custom levels because the game was never coded to sufficiently distrust inputs
net-wireless/spectools Build the raw output client for spectools. (for developers)
dev-libs/boost Build and install only the debug version of the Boost libraries. Only enable this flag if you're developing against boost.
dev-util/google-perftools Build a set of libraries with debug support (so-called debugalloc). These are available by default but are not needed unless you're actually developing using tcmalloc.
dev-libs/openct Add debug output to the driver library for pcsc-lite.
www-client/chromium Enable DCHECK feature with severity configurable at runtime. Mostly intended for debugging and development, NOT RECOMMENDED for general use.
media-tv/mythtv Instructs Qt to use the 'debug' target
net-misc/chrony Get DEBUG_LOG output from chronyd when passing -dd parameter
net-mail/vchkuser Enables debug messages to syslog
sys-auth/pam_u2f Enable debug messages using the pam logging macros.
sys-libs/glibc When USE=hardened, allow fortify/stack violations to dump core (SIGABRT) and not kill self (SIGKILL)
www-servers/monkeyd Enable lots of debugging info
x11-wm/e16 Build libtrip debugging library

All packages providing a “debug” USE flag (1397)