A curable composition comprising: cationically curable compounds; and a cationic-polymerization initiator, the cationically curable compounds comprising: an epoxy compound in an amount of 10 to 90 weight percent of the total weight of the cationically curable compounds, the epoxy compound comprising a compound represented by Formula (a); and an oxetane compound in an amount of 5 to 40 weight percent of the total weight of the cationically curable compounds, the cationic-polymerization initiator comprising an onium borate salt comprising an anionic moiety represented by Formula (c-1) and a cationic moiety represented by Formula (c-2), Formulae (a), (c-1), and (c-2) expressed as follows: wherein R 1 to R 18 are each, identically or differently, selected from hydrogen, halogen, a hydrocarbon group optionally containing oxygen or halogen, and optionally substituted alkoxy; and X is selected from a single bond and a linkage group, [ k B(Phf) 4-k] - (c-1) wherein Y is, independently in each occurrence, selected from C 6 -C 30 aryl and a C 4 -C 30 heterocyclic group, each of which may have one or more substituents (excluding halogen-containing groups); Phf represents, independently in each occurrence, a group resulting from replacing at least one of hydrogen atoms of phenyl with at least one selected from the group consisting of perfluoroalkyl, perfluoroalkoxy, and halogen; and k is an integer of 1 to 3, wherein "A" represents an m-valent atom selected from sulfur, iodine, and selenium (Se) atoms; R a, R b, and R c are monovalent groups and are each, identically or differently in each occurrence, selected from a C 1 -C 30 hydrocarbon group, a C 4 -C 30 heterocyclic group, and a group including two or more of these groups bonded to each other through a single bond or a linkage group, each of which groups may have one or more substituents; R d is a divalent group and is selected from a C 1 -C 30 hydrocarbon group, a C 4 -C 30 heterocyclic group, and a group including two or more of these groups bonded to each other through a single bond or a linkage group, each of which groups may have one or more substituents, where the linkage group is, independently in each occurrence, selected from -O-, -S-, -SO-, -SO 2 -, -NH-, -NR'- (where R' is selected from C 1 -C 5 alkyl and C 6 -C 10 aryl), -CO-, -COO-,-CONH-, and C 1 -C 3 alkylene; and n is selected from 0 and 1. cationically curable compounds; and a cationic-polymerization initiator, the cationically curable compounds comprising: an epoxy compound in an amount of 10 to 90 weight percent of the total weight of the cationically curable compounds, the epoxy compound comprising a compound represented by Formula (a); and an oxetane compound in an amount of 5 to 40 weight percent of the total weight of the cationically curable compounds, the cationic-polymerization initiator comprising an epoxy compound in an amount of 10 to 90 weight percent of the total weight of the cationically curable compounds, the epoxy compound comprising a compound represented by Formula (a); and an oxetane compound in an amount of 5 to 40 weight percent of the total weight of the cationically curable compounds, the cationic-polymerization initiator comprising an onium borate salt comprising an anionic moiety represented by Formula (c-1) and a cationic moiety represented by Formula (c-2), Formulae (a), (c-1), and (c-2) expressed as follows: wherein R 1 to R 18 are each, identically or differently, selected from hydrogen, halogen, a hydrocarbon group optionally containing oxygen or halogen, and optionally substituted alkoxy; and X is selected from a single bond and a linkage group, [ k B(Phf) 4-k] - (c-1) wherein Y is, independently in each occurrence, selected from C 6 -C 30 aryl and a C 4 -C 30 heterocyclic group, each of which may have one or more substituents (excluding halogen-containing groups); Phf represents, independently in each occurrence, a group resulting from replacing at least one of hydrogen atoms of phenyl with at least one selected from the group consisting of perfluoroalkyl, perfluoroalkoxy, and halogen; and k is an integer of 1 to 3, wherein "A" represents an m-valent atom selected from sulfur, iodine, and selenium (Se) atoms; R a, R b, and R c are monovalent groups and are each, identically or differently in each occurrence, selected from a C 1 -C 30 hydrocarbon group, a C 4 -C 30 heterocyclic group, and a group including two or more of these groups bonded to each other through a single bond or a linkage group, each of which groups may have one or more substituents; R d is a divalent group and is selected from a C 1 -C 30 hydrocarbon group, a C 4 -C 30 heterocyclic group, and a group including two or more of these groups bonded to each other through a single bond or a linkage group, each of which groups may have one or more substituents, where the linkage group is, independently in each occurrence, selected from -O-, -S-, -SO-, -SO 2 -, -NH-, -NR'- (where R' is selected from C 1 -C 5 alkyl and C 6 -C 10 aryl), -CO-, -COO-,-CONH-, and C 1 -C 3 alkylene; and n is selected from 0 and 1.