Read though your bash startup scripts (bashrc, profile, bash_login) for a line that tests for an aliases file and runs it if it exists. You can create that file if it doesn't exist. Mine looks for ~/.aliases.
If it doesn't exist, you can add alias definitions to ~/.profile or ~/.bash_login whichever your system uses.
Lately, distros have system shell startup files in /etc/profile.d/.
On my system, my aliases are defined in /etc/bash.bashrc but since this file can be replaced in an update, /etc/bash.bashrc would be used for changes instead.
You could grep files in /etc/ looking for an alias you have.
grep "alias md='mkdir -p'" /etc/* 2>/dev/null