Real Dictionary

lanthanum in American English
a soft, malleable, silvery chemical element, one of the rare-earth elements and the first member of the lanthanide series: symbol, La; at. no., 57
noun: a rare-earth, trivalent, metallic element, allied to aluminum, found in certain minerals, as monazite; at. wt.: 138.91; at. no.: 57; sp. gr.: 6.15 at 20°C

lanthanum in British English
noun: a silvery-white ductile metallic element of the lanthanide series, occurring principally in bastnaesite and monazite: used in pyrophoric alloys, electronic devices, and in glass manufacture. Symbol: La; atomic no: 57; atomic wt: 138.9055; valency: 3; rela