Dielectric, capacitor, electrical circuit, circuit board, and apparatus

A dielectric comprising a composite oxide including at least two selected from the group consisting of Nb, Bi, and Ta, wherein;

• the following requirement (I), (II), or (III) is satisfied:
• (I) the composite oxide includes Nb, Bi, and Ta;
• (II) the composite oxide includes Nb and Bi and is amorphous;
• and
• (III) the composite oxide includes Bi and Ta and is amorphous,
• the composite oxide has composition represented by NbxBiyTa(1-x-y)Ok,
• in the composition, k is a value for maintaining electroneutrality of the composite oxide, and
• the composition satisfies the following requirement (i) or (ii):
• (i) x≥0.70, 0<y≤0.3, and 0<x+y<1;
• and
• (ii) x<0.70, 0.10≤y≤0.90, and 0<x+y<1.