A straight bond is a bond the principal of which is repaid to the lender by the borrower in full when the bond reaches its maturity date at the end of the bond term.
This term is synonymous with bullet bond. The majority of bonds issued by companies (corporate bonds) and governments (government bonds and municipal bonds) fall under this category.
Bonds which do not fall under the straight bond category include perpetual bonds (which do not have terms and never mature) and amortized bonds (the principal of which is repaid via regular amortization payments throughout the bond term).
More on this topic:
Swiss online broker comparison