155 (one hundred [and] fifty-five) is the natural number following 154 and preceding 156.

155 is:

• a composite number
• a semiprime.
• a deficient number, since 1+5+31=37<155.
• odious, since its binary expansion has a total of 5 ones in it.

There are 155 primitive permutation groups of degree 81. OEIS: A000019

If one adds up all the primes from the least through the greatest prime factors of 155, that is, 5 and 31, the result is 155. (sequence A055233 in the OEIS) Only three other "small" semiprimes (10, 39, and 371) share this attribute.