The term 'stiffen' can be defined as to become or make rigid, firm, or hard. There are many words that can be used to describe this process, such as THICKEN (7 letters), CURDLE (6 letters), PETRIFY (7 letters), OSSIFY (6 letters), CONGEAL (7 letters), CAKE (4 letters), CLOT (4 letters), COAGULATE (9 letters), SET (3 letters), HARDEN (6 letters), STARCH (6 letters), GEL (3 letters), TENSE (5 letters), and tenseup (7 letters). Each of these words describes a process in which something becomes more rigid, firm, or hard.