恒温器[kouonki] incubator
恒久[koukyuu] permanent
恒久化[koukyuuka] making permanent
恒常[koujou] constancy
恒心[koushin] steadiness

恒星[kousei] (fixed) star (i.e. not a planet, meteorite, etc.)
恒等式[koutoushiki] identity
恒例[kourei] established practice
恒河沙[gougasha] 10^52 (or 10^56)
恒数[kousuu] a constant (e.g. in science)