A "bicycle" is a form of straight in poker where the ace is treated as a one instead of an ace. Where the hand shows two, three, four, five ace, a player has a bicycle straight with the ace being the low end of the straight.
Most amateur players overestimate the chance of getting a bicycle. It is not uncommon to see people call raises, or even re rasing with hands like three, five suited, hoping to make a bicycle. However sometimes these players are smarter than you estimate. Usually a raise is based upon a player holding an ace. If an ace hits, it is most likely that the player will bet large or go all in. However if the flop shows both a straight draw and a flush draw, the player holding the cards with little value at the outset will win big, as the better player assumes that no sensible person would call his raise with cards that would fit with the low end of the board.
In reality it's harder to make a bicycle than a regular straight because the ace can only be the low card.
A bicycle is also known as a "wheel or a "bike"
By its very nature, a Texas holdem straight is defined as a five-card poker hand in consecutive order from lowest to highest. This includes using the ace to have the lowest value that's unique in this poker hand rank. In effect, the wheel or bicycle (ace to five, unsuited) is the lowest and worst sequence possible in this hand rank. Broadway (ten to ace, unsuited) is the highest in this hand rank of unsuited cards. And given those two ends of a Texas holdem straight, the in-between sequences include five-card poker hand series making the six-high, seven-high, eight-high, nine-high, ten-high, jack-high, queen-high and king-high straights in the game.